Planning a garden means more than just sending off for seeds from catalogues. A couple of steps taken now will pay off later with a better, more organized garden.

The first step is a soil analysis to determine what nutrients and minerals are in your garden ground. County extension agencies will tell you how to collect a sample and send it in for analysis. Soil testing costs $4 per sample in Maryland and Washington but is free in Virginia.

Most extension offices have special kits, generally a bag or box with instructions on what to put in it. In Fairfax County and Alexandria, you can pick up and return kits at all libraries and at the extension offices. In Arlington, kits are available through the extension office only. In Montgomery and Prince George's counties, you must write or call the extension office to have a kit sent out. The sample is then returned to the University of Maryland for testing.

Extension office kits should not be confused with commercial kits for testing soil yourself. The commercial kits include small bottles of various testing agents that are supposed to provide an accurate soil analysis. But the extension tests are generally more reliable than home versions. They're analyzed in specially equipped labs by experienced technicians. The analysis usually is then sent to an extension agent for suggestions on improving the soil. And finally, the results and recommendations are sent to you. The whole procedure takes about two weeks at this time of the year but may take a month during the spring rush. (In the District, count on a month.)

Taking soil samples is simple, according to several county agents. Don't try it, of course, when the ground is frozen or very wet. Take the samples from all over the garden (some agents suggest four to six locations, others eight to 10). Take some samples from the soil surface, others down an inch or two, others still farther down to a maximum of six inches. Put the soil in a clean bucket -- without even a trace of fertilizer, lime or chemical. Then mix the four to 10 samples thoroughly and put one cup of the mixture in the kit bag or box.

Once you've taken the samples, be sure to fill out all forms that accompany them to the lab. You'll be asked such questions as what you intend to plant and what's been planted in the past.

How often should you take soil samples and from where? A healthy, rich vegetable garden doesn't need to be tested more than every three years. But one that isn't so rich or isn't producing large, fast-growing plants and vegetables should be tested annually so that the soil can be improved each year. If part of your garden is fairly old and another part is new and less fertilized, take two samples. If different parts have different-colored soil (aside from differences owing to depth) keep the samples separate and have two or more analyses done.

Soil tests are useful not only for vegetable gardens. If you're unhappy with your lawn, send in a sample. If you're planning a new flower bed, take samples before you start it. If you've never had a soil test on your property, do it now and give yorself a base from which to start building up your soil's health.

Additional information on soil samples is available from these county extension offices: Howard, 997-7878; Montgomery, 948- 6740; Prince George's, 952-3226; Fairfax, 691-3456; Alexandria, 838-4333; Arlington, 558-2475; Loudoun, 777-0373; Prince William, 369-9262. In the District, call 282-7400.

Now that your soil is analyzed, how should you organize your garden so that you know how much and what kinds of seed to buy? That's where computerized garden planning can help. And Brookside Gardens is one place you can get help from a computer.

You simply provide basic information from a checklist -- the kinds of vegetables your family eats, how many people you feed, whether the produce is to be used fresh or processed and so on. The computer plans gardens ranging from a foot square to 900 feet square and specifies the best vegetables for this area. The results are mailed back to you along with a planting guide -- a "guide to the placement and quantities of vegetables by row, cultural notes and garden tips," all for $4. For more information, write: Brookside Gardens, 1500 Glenallan Avenue, Wheaton, Maryland 20902.

A similar program has been developed by Purdue University and is available through a seed company for $3.95. Write to Smarter Garden Plans, Northrup King Co., Box 1615, Mineapolis, Minnesota 55440.