The biggest mistake beginning gardeners make is using lousy or too-thin soil. Before planting anything in your yard, prepare your garden beds by digging to loosen the soil and adding organic material! This prep work can save you untold disappointment and, perhaps more than any other factor, assure a bountiful and delicious harvest.
If you're working with a brand-new garden (or one that fell fallow and you're bringing it back to life), you can stake it and get it ready the autumn before you plan to plant. This act gives the soil and the amendments you've added time to settle and meld. It also means you have less work to do next spring.
If a fall start isn't possible or practical, go ahead and prepare the ground in spring — but don't start too early. If the ground is still semi-frozen or soggy, digging in the soil can compact it and harm its structure. How do you tell whether it's ready to be worked in? Grab a handful and squeeze — it should fall apart, not form a mud ball.
Follow these steps when preparing your soil:
Most plants are content with 6 to 8 inches of good ground for their roots to grow in.
If you're planning to grow substantial root crops (potatoes, say, or carrots), go deeper still — up to a foot or more (yes, you can use a technique called hilling, where you mound up good soil around crops like potatoes, but this method doesn't excuse your making a shallow vegetable garden).
Fill 'er up.
Add lots and lots of organic matter! Try using compost, dehydrated cow manure, shredded leaves, well-rotted horse manure (call nearby stables), or a mixture. If your yard happens to be blessed with fertile soil, adding organic matter is less crucial, but most soils can stand the improvement. Mix it with the native soil, 50-50, or even more liberally.
Maybe your area's soil is notoriously acidic, or very sandy, or quite obviously lousy for plant growth. The good news is that organic matter can be like a magic bullet in that it helps improve whatever you add it to. You have to replenish the organic matter at the start of every growing season or maybe even more often. (If the soil stubbornly resists improvement, resort to setting raised beds atop it and filling these bottomless boxes with excellent, organically rich soil.)