OK, you've figured out what your space is and evaluated your soil. You've started working toward improving your soil and giving your garden definition. Right? You've been paying attention haven't you?!
Now it's time to decide what you're going to grow.
Let's assume you're not going to be putting 1/2 an acre under the plow and you're just doing a small backyard garden. With that in mind, you need to plant species and varieties that make efficient use of space. . . ex: potatoes require lots of space to get a high yield. Also, potatoes are relatively cheap if you buy 50# bags in the fall. So if your space is limited forget about things like potatoes. I feel the same way about broccoli. Forget about corn also. Two ears per plant just isn't worth using valuable garden space for if you're trying to be productive.
So what's good in a small space? Tomatos are great. Grow them close together and vertically on stakes and you get a high yield veggie that is easily canned or frozen. Six to a dozen plants will keep you in tomatos and sauce all year long. The real champion of product per square foot of space is bush beans. Beans grow profusely. A 20' row of beans will yield lots of fresh eating and some for freezing and canning. Two 20' rows will keep you in beans all year (if you're only feeding a family of three or four). I love fresh beans. Blue Lake bush have always been my preferred bean variety. When it comes to beans, I prefer not to go verticle and pole beans and half runnings tend to yield less than bush.
Another good crop which isn't quite so prolific is peas. You can grow them on a fence along the edges of you garden so they don't take up any space and there is simply nothing more delicious and sweet than a fresh raw pea put of the pod. Again, vertical is always good in a garden.
Cabbages, if planted early enough are good crops. Plant them close to each other, mulch and fertilize well and you'll get a lot of cabbages in a fairly small space. Store them in a cool dry place and they'll last well into the winter. Make saurkraut and they'll last forever. When you harvest the cabbages in early to mid summer, you may be able to use the spave to plants something else like beans.
Like tomatos, I get good yields from egg plants. Some things you'll have to buy plants and others you'll be able to plant seeds. Check things out at your local nursery/garden center.
One last recommendation. . . SQUASH. Summer bush varieties will give you very high yields and not take up too much space.
The best thing to do is to get some seed catalogs and read them. You can order free on-line from many different seed companies. Go google.
Remember, gardening lasts well into fall. You can plant early spring crops in early fall and harvest right up till the first frost and beyond. I've picked brussel sprouts at Christmas. Spinach will grow past the first frost but not in the heat of summer. Peas also do well if you time them right and get a late frost.
The best thing to do is go down to your local nursery on a day when they're not busy (don't go on Sat. afternoon. . . try Wed. morning) and start talking. Good gardeners love to share their experiences and people who work in garden centers love to give advice. Go around to every place in your area and tell them about your garden and what you want to do with it. You'll get more valuable information that way than reading my bullshit on the internet.
1) Plan and define your area
2) Prepare your soil
3) Decide what to plants you want and learn about them