For a man who is in the business of giving tax advice, it may be an unusual position. But former Internal Revenue Service agent Barry R. Steiner is convinced most Americans can fill out their own tax forms without hired help.

"anyone of reasonable intelligence," says Steiner, a John Belushi lookalike, "should try to do their own income tax" -- unless, he adds as a proviso,"you happen to be a client of mine in Chicago." The IRS says about 60 percent of taxpayers do complete their own returns

If you get most of your Form 1040 filled out and still have a question or two, he suggests using a little chutz-pah. "Look in the Yellow Pages under Certified Public Accontant for five sympathetic names. Call them on the phone and say: 'I have a problem. Would you mind if I took a minute of your time?"

"one guy," he insists, "will answer your question free. It'll work -- if you have only one or two points, not 25."

Won't that anger CPAs? "I'll answer. I do this quite a bit. If I appear to be on a mission, I'm on a mission. I get my jollies by helping people save money."

Another suggestion is "to get a hold of a good income tax book -- mine or someone else's." Steiner, a CPA himself, is author of "Pay Less Tax Legally: The Tax Preparation Guide to the Latest Laws and Loopholes" (Signet, 162 pages, $3.95 paper). That cost, incidentally, "is tax deductible."

What you should not do is get advice "from your brother-in-law. If he's not a CPA, he doesn't know what he's talking about." Nor does he think the IRS phone-in service is very helpful. "You call three different people and you get two to three different answers."

You probably should seek help, he believes, if you:

"earn $50,000 and have rental property or a tax shelter.

"have a small business. You're better off not doing your own payroll taxes.

"are a salesperson on the road with agreat deal of travel expense. There are certain options you may not know.

"have anything that's unusual or complicated, like a divorce."

Steiner, 37, spent three years with the IRS after graduating from De Paul University in 1964. "I was working for an accounting firm, and saw auditing as quite boring. The only area that interested me was income tax. The one place to learn was the IRS."

Something else he learned: "The IRS is never out to help you save money."

With this experience, he began writing a tax advice book. The first edition he published himself. Since 1975 it has been a Signet book, updated annually. It has sold, he says, "1 million" copies.

Steiner makes two strong arguments in the book, which is aimed at the middle-income American earning $8,000 to $50,000 annually:

First, he writes, "Ginuine tax savings result from adquate tax planning. The secret of cutting your tax bill coames from thinking about tax impact and consequences before entering into a personal or business transaction that may affect your taxes."

That means, in many cases, it's too late now to achieve big savings on the return you have to file by April 15. You should, instead, start planning now for April,1982, "Too many taxpayers mistakenly believe that real savings come from locating some mysterious new deductions or tax loophole."

Among his tax planning strategies:

generating tax-free income, from such things as interest on municipal bonds. "How much tax-free income you need depends largely on your tax bracket."

claiming everything you are entitled to.

dropping a tax bracket or two. He suggests income-averaging if your income makes a big jump one year or "dividing income-producing property among family members with trusts."

deferring the tax, by holding off taxable income if lit helps your situation.

His second argument is that you be "aggressive" in your tax planning.

Too many tax advisers, he believes, take "an extremely conservative position," which costs their clients money. "You have better uses for your money that giving it to Uncle Sam."

Being aggressive can pay off, he found out while still in high school. His father, as he tells it, one year claimed a sales tax deduction of $720 based on a standard tax table. But inadvertantly, he entered the figure as $720 on the form. When the IRS called him in for an audit, "they compromised at $600." f

The moral he got from this: "If you don't claim it, they're not going to give it to you."

Because of the complexity of the tax laws, "You can take the identical figures to six offices." Depending on how they handle them, the differences in saving can amount to "50 percent."

A cautionary note: "Aggressive tax planning involves taking some risks that can go sour." But "the worst thing that can happen is that you may have to give back some taxes . . . plus a little interest.

His advice: "It's better to shoot high and fail than never to try at all."