If you are one of the 9 million people covered by the Federal Employe Health Benefits Program -- that includes about half the population of metro Washington -- you must make an important personal and financial decision very soon.

Monday, May 3, is the start of the "open season" period (it ends May 28) when federal and postal employes and U.S. retirees decide whether to stick with their present health plan, or shop around for better (or cheaper) protection for the remainder of this year.

Insurance premiums for workers and retirees jumped an average of 30 percent in January. Most plans reduced, or eliminated, certain kinds of coverage. But workers have been stuck with the higher-cost, lower-payout plans because the Office of Personnel Management postponed the open season -- supposed to be held last November -- for picking 1982 health plans. It begins next Monday.

There are 120 plans in the FEHB. But many of them are regional, or limit coverage to certain groups. For all practical purposes, the average federal worker has between 15 and 20 plans from which to choose. Some of the best are little-known, union-backed programs. Nonmembers must pay a special fee to join them.

The picture is confusing. This open season, because of high premiums and lowered benefits, is the most important ever.

If you switch plans in May, coverage under your new plan will not begin until July (July 1 if you are a retiree or are paid monthly, or July 11 for most other workers).

Open season shopping will be confusing. This is what you will need:

* A copy of the 12-page rate book put out by the Office of Personnel Management. It shows premiums (high and low option) for each plan, and compares them to 1981 rates. Copies of the free booklet are supposed to be handed to each employe in early May. (A separate book that makes comparisons based on monthly premiums has been sent to retirees.)

* A copy of the 36-page benefit book from OPM that lists significant benefits offered by each of the health plans. It is free.

OPM says that nearly 8 million copies of the booklets listing rates and benefits have been printed. Retirees have been sent information on their current health plan, with a tear-off post card they can use to request information on other plans.

If you do not get the booklets at work by the end of next week, OPM says to check with personnel or your health benefits people. They have, or are supposed to have, enough copies to give each individual.

Retirees who want information on other plans can write to Open Season Task Force, P.O. Box 14172, Washington, D.C. 20044.

Three important points:

* There will not be a transfer fee for persons who switch from one plan to another.

* Preexisting medical conditions (except for certain cosmetic and dental care) will not be excluded by your new plan.

* Although individuals switching health plans will have to meet deductibles required by their new plan, amounts already paid to satisfy deductibles will be credited toward the deductible of the new plan.

Once again, open season runs from May 3 to May 28. If you do change plans, your new coverage will begin in July.

There will be another open enrollment period in November of this year for health coverage for 1983.

Rating the Health Plans: Washington Consumers' CHECKBOOK guide to FEHB plans is out this week with its hard-nosed matchup benefits and costs plan-by-plan. It claims feds can get top protection, and save up to $400, by shopping carefully.

The highly regarded guidebook suggests people check out HMOs (Health Maintenance Organizations) and some union-backed plans. The 58-page guide costs $3.95. For details call 347-9612 or write to 1518 K St. NW, Room 406, Washington, D.C., 20005.