Answer: It is always better to start out small and add to a garden than to start out too big with more than you can handle and become frustrated.
Tomatoes and peppers are fairly easy to grow, as are pole beans, bush beans, radish, greens, beets and herbs. Eggplant has many pests and may be difficult as a result, but otherwise its culture is similar to that of tomatoes and peppers. Corn is problematic in that it takes a large area to grow enough to get good pollination, so with only 20 x 40 feet I would suggest you skip that particular crop. You might want to look at a book on small space gardening such as "How to Grow More Vegetables in Less Space Than You Can Imagine" by John Jeavons or Mel Bartholomew's "Square-Foot Gardening" for pointers on layout, crop rotation, and soil preparation.
Some easy annuals to grow in full sun are marigolds, zinnias, petunias, nicotiana, ageratum, sunflowers, morning glories, cleome, strawflowers, salvia, and dahlias. Perennials to consider might include black-eyed Susan, purple coneflower, veronica, perennial salvia, Siberian iris, daffodils, crocus, tall phlox "David", ornamental grasses and sedums of all kinds.
Have fun with your garden!
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