Make sure you grow beans.
Regardless of what you put in your garden, make sure you have various types of legumes spread out throughout the garden. Beans put nitrogen into the soil, which every other plant needs.
As far as everything else goes, once you get your soil put together, do a pH test before you plant anything. Some plants (onions, potatoes, garlic, e.g.) require a lower pH than others, and you might have to acidify the soil to grow them properly.
Certain plants should not be grown from seed in your garden. Tomatoes and peppers are good examples. They should be started indoors in a temperature and humidity controlled area (I cover the starter pots with saran wrap after watering them), and then transplanted. If you don't want to go to that trouble, just buy young plants later in the season.
If your soil is sandy, you'll have problems keeping it moist. Certain plants that crawl on the ground with broad leaves (think squash) will help hold in moisture. As will mulch and hay. In fact, putting a small layer of hay over your garden right after planting seeds and watering is a great way to not only hold in moisture, but also give your seeds a head start before weeds can take root.
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