CONGRATULATIONS ON your nomination to head an important federal agency. Supposedly you are about to manage hundreds or thousands of employes and millions or billions of dollars. Supposedly you will be helping President Reagan make national policy more responsive to the people. No doubt you are looking forward to dining at the White House, to sipping champagne from the secret icebox in the Presidential Box at the Kennedy Center, to gala evenings at Wolf Trap. Before you pack, however, there are a few facts you should know.

Start thinking about a $175,000 mortgage at 13 1/2 percent. Think, too, about the cost of frequent entertaining, and about spending $3,000 per year per kid on schooling. It ought to be easy on a Cabinet officer's $69,630 -- unless you want to eat, too. In that case, you'd better have a little coming in on the side.

Speaking of outside income, be sure to comply with the Ethics in Government Act, which requires disclosure of everything your father-in-law has given the wife and kids, conflicts of interest arising out of past or future employment, and conflicts of interest -- real, potential, and imainable -- flowing from your investments. Oral guidance on how to handle your assets will come from the White House staff, the Office of Personnel Management and the agency you have been nominated to oversee. But no amount of guidance can guarantee that your carefully constructed portfolio won't self-destruct, though you may rest assurd that your broker is about to make some big bucks.

After surviving the confirmation process and taking office, you will want to review your budget for fiscal 1981. Alas, it is fixed; you have practically no authority to reprogram funds. If you want any changes, even changes developed in conversation with President Reagan himself, you must get permission from one of the four agencies that are the center regulatory mechanisms of the federal government.

Central agency No. 1 is the Office of Management and Budget. OMB staffers -- Washington pros call them micromanagers -- often try to prove their mettle by nickel-and diming your budget until it meets their notions of what it should look like. In dealing with OMB, you should by-pass the staffers, whose pretensions exceed their authority, and deal with OMB's directors. Let your subordinates play onesy-twosy with the OMB subordinates, which should lessen the chances that either group will make trouble for you.

Now suppose that you propose changes in the 1981 budget, and that they emerge in recognizable form from OMB. You're home free? Hardly. The gantlet of congressional committees and subcommittees remains to be run.

Each member of each committee and subcommittee serves a different constituency and therefore is sharpening a different ax -- but most of the axes will be aimed at your programs. Moreover, each committee and subcommittee has its own staff, whose members mean to micromanae as much as OMB staffers.

For example, with a budget of $10 billion, the secretary of energy cannot reprogram one dollar without the approval of the staffers of the House subcommittee on appropriations. One of them formerly served as controller of the Department of Energy, a post he left after a difference of opinion with the prior secretary. He now helps run the department from his subcommittee office on the Hill. Welcome to Washington!

Your current agency budget is cast in concrete, so you turn to next year's budget to discover that you're too late again. Fiscal 1982 has already gone to Congress, which means that, come February, you're going to have to defend a budget prepared by your predecessor.

In order to amend the 1982 budget, you will again have to face the committees and subcommittees. Preparation for testimony will run at the rate of four hours of homework for each hour of scheduled apearance, and you may have to appear 10 or 15 times. Following your testimony, congressional staff will issue questions requiring detailed written response. Answering will take more time than preparing the original testimony.

After resolving budget issues, you turn to the future. What plans does your agency have, you wonder. For an answer, you search out the members of your agency's long-range-analysis division. When you figure out who they are, they're apt to be attending seminars sponsored by Brookings or the American Enterprise Institute on subjects like "How to Convert Performance Budgeting to Program Budgeting to Planning Programming Budgeting to Management by Objective to Zero-Based Budgeting." You realize that you'll be long gone before any planning efforts reach fruition.

Personnel, you think. Maybe there's hope in personnel.

Just now oficial Washington is in a monumental uproar over the Senior Executive Service program, established by the Civil Service Reform Act of 1978. The program, which applies to the top 8,100 civil servants, authorizes bonuses for good performance and dismissals for underachievers. After the first 120 days of Reagan's term, the law will let you "involuntarily reassign" your top career people. Naturally they're paralyzed with fear that you will do so.

You will be tempted to take advantage of this historic opportunity, but be careful. Choosing whom to get rid of will soak up time just when you should be focusing on policy direction. And senior civil servants, once on the outside, can make a lot of legal and political trouble for you.

Dealing with personnel will put you in contact with central regulatory agency No. 2, the Office of Personnel Management, which has formidable powers of resistance. So do your equal employment opportunity director, your affirmative action officer, your minority affairs officer and your federal women's program action official. You'll also get to know the director of your Office of Small and Disadvantaged Business Utilization, who oversees your use of the procurement function as a "social tool."

The list of goals monitored by these officials is endless. They include a procurement-dollar set-aside for somen-owned companies; a 25 percent hiring goal for minorities; a 25 percent hiring goal for professional women, and a 25 percent promotion goal for promotions above GS9 ($18,585 a year) for minorities and women.

You'll also contend with central agency No. 3, the General Services Administration, which allocates office space, maintains buildings and provides word-processing equipment. GSA controls the temperatures in buildings, the flow rate of hot water and the number of usable bathrooms. Its rules specify whether employes should wear ties, who will have which parking spaces and what the spaces will rent for.

About the only thing the General Services Administration is not concerned with, it seems, is services. Many federal buildings are dirty. Furniture is often old and worn. Poor security is a constant worry. The executive time wasted in negotiations with GSA over such problems is incalculable.

Finally, you'll want to get ready to deal with central agency No. 4, the General Accounting Office. In all probability, a GAO auditor will be situated within your agency. So while congressional and OMB staffers are micromanaging your agency, a GAO employe, also responsive to Congress, will be independently reviewing the whole process. GAO investigators routinely follow up on past GAO recommendations. Though such recommendations may become outdated, your friendly neighborhood inquisitor will keep nagging to find out what you've done, and you may spend more time answering the pursuers of truth than you do managing your agency.

You will be tempted to delegate supervision of GAO-related activities to one of your assistant secretaries, but that would be a mistake. You will be the one featured in news stories alleging "waste and derelictions" based on GAO reports.

Then there's "son of GAO," the inspector general, who is responsible for auditing and investigating your agency's operations. This new office, created in 1978, derives from the military. But whereas the military IG reports to the unit commander, the federal IG, though recommended by you and appointed by the president, is confirmed by Congress.

No one knows whom the IG really works for. No matter: With everything open to his scruitiny, the IG will provoke fights among your subordinates over whose programs should be investigated. With a friend like the IG, you will wonder, who needs enemies?

I have saved two critical items for last: your relations with the press and with the White House staff.

Never forget that an adversary relationship will always exist between you and the Washington press, which today consists of hundreds of graduates of the Woodward and Berstein School of Journalism. So think defensively. Hire a first-rate PR man in whom you have 100 percent confidence, and follow his advice. Never meet with reporters unles your press secretary is present.

The best measure of your success with the press will be no press. For every column inch of coverage you receive, your standing with the White House will go down. In the president's eyes, the presss is his press. You are on his team, and team players need no press.

The White House staff presents the toughest challenge, and another adversary relationship. Dealing with it will test your survival instincts to the utmost.

Not a few White House staffers will have earned their jobs by serving as political baggage handlers, ticket clerks and microphone checkers. Others come from academe, where they never had even a secretary working for them.

Naturally, White House staff members think you work for them. In your judgment, of course, you are working for the president. My advice is: Trust your judgment. The president, not his aides, talked you into the job. If at that meeting it wasn't made clear whom you would report to, you are in trouble.

Still, the power of the White House staff increases each day. The president's men have greater access to information on broad issues and goals, while you are bogged down with bureaucratic trivia. And don't forget: They are keeping book on your mistakes.

In short, you are about to become a fulltime budget examiner, personnel analyst and EEO adjudicator. You are not really going to be a manager. Federal executives have long since been deprived of the time to think through problems, plan, give policy direction and control their agencies.

Thus, as the months go by, one of three things is going to happen.

First, your calendar becomes overscheduled, your managerial talents are underutilized, ulcers or family problems crop up and you begin to burn out. A recent survey found that only half of Carter's appointees got past the first three years.

Second, seeing that you cannot really manage your agency, you decide instead to advance your postgovernment employment opportunities. You've got two years, maybe three, to make the right contacts. Enjoy the title, the staff, the blanker travel orders and all the other fringe benefits of your job. But kid yourself not about the nature o your role or the size of your contribution because you will surely kid no one else.

Or, third, you can relax, since you may not really be qualified for the job anyway. Increasingly, the federal government is run by people with marginal skills. Productivity and efficiency slowly dwindle, and budget overruns and inability to manage critical programs are coming to typify the federal bureaucracy.

Surely, you object, the sytem can be beaten. And it's true that some guys do beat the sytem. Let me give you four names: James r. Schlesinger, Joseph A. Califano Jr., George P. Shultz and the survivor of survivors, Elliott Richardson. All had a realistic sense of the bureaucratic context; all refused to become hostages to trivia; all established direct access to their presidents; and all retained enough independence to leave the system rathern than be beaten by it. But except for Shultz, all of them ended up being fired.

As I said, welcome to Washington.