There are good reasons for starting plants for your garden from seeds you sow indoors youself. Sometimes it is the only way to get the kinds of plants you want. You can also save money and get pleasure out of doing it. It is possible to get anywhere from a three- to eight-week advantage over outdoor seeding.

An informative new book, "Park's Success With Seeds," by Ann Reilly, can take a lot of the guesswork out of indoor planting, with 364 pages and more than a thousand full-color pictures of the vegetables and flowers described. The book costs $9.95 until July 31, 1979, then $12.95, and can be ordered direct from the publisher, George W. Park Seed Co., Greenwood, S.C., 29647.

"Our horiticulturists search throughout the world for the newest and finest variety of seeds," says William J. Park, president of Park Seed Co. "Many of these are unusual and little-known species for which there has been no readily avaiable publication providing instructions for their germination and care."

The problem for most gardeners with starting seeds indoors is light. All seedlings need ample light to develop into strong, Healthy plants. Seedlings have the highest light intensity requirements of all plants. An unshaded south windowsill is good, but a lot of cloudy days without sunlight may play hob with them.

Fluorescent ights can provide the necessary light reliably, easily and inexpensively. Seedlings can be put anyplace -- under shelves, on bookcases or kitchen counters.Put them in full view in one of the living areas, or hide them away in attics, basements, closets or spare rooms. Reilly's book provides adequate information on use of fluorescent lights.

There is also information on containers used to grow the seeds in, on soil and soilless mixtures, on how to sow the seeds -- in fact, on everything you need to know to be successful.

"I did my share of things that children did when I was growing up in Brooklyn -- bicycle-riding, roller-skating, jumping rope," says author Reilly. "But unlike other children, I passed many hours in our small back yard plainting nasturtiums or radishes or watching (and trying to help) my aunt with her garden.

"When I was in college, in the days before the houseplant boom, I was the only person in the dormitory whose room was decorated in greenery. Once I owned my own garden, I grew everything I could beg, borrow or buy.

"The world has caught up with me, for now most everyone has a garden, even if it is only on a windowsill. House plants have become a part of the decor, and vegetables are grown from terraces to empty lots to combat the rising cost of food. People, in their return to nature, are realizing the beauty of the world of flowers."

It is possible to collect your own seeds from wooded areas, roadsides or your flower garden, Reilly says. But your results probably won't be as good as with purchased seeds. Your seeds probably will not produce plants identical to the parental plants.

Commercially grown seeds are carefully rouged to keep quality high. They are harvested at maturity, properly collected, cleaned, processed and stored in the ost modern ways so that they are the best possible seeds you can get anywhere.