Pick your plants.
Before you plant anything, think about what you want to grow, then research the plants you want. Do they need started indoors from seed? Would you rather purchase them as a starter plant? Are they able to be sown directly into the soil? Some seeds are frost tolerant (such as collards and spinach) but most are not. SparkPeople's Backyard Gardening Guide lists these stats and more for a wide variety of fruit and vegetable plants, so that's a great place to start. You may need to narrow down your list based on the plant's needs and your available space. By the end of this step, you should know which plants would grow well in the space you have available.
Next, think about what kind of yield would be useful for your diet and family size. It can be very tempting to sow every seed in a packet, but for many varieties, it can be too much. Consider lettuce for example. A one-ounce package of seeds contains around 16,000 lettuce seeds in it! Thinning and failed germination is to be expected, so try to strike a balance between those factors and how much you actually consume. A packet can always be sealed up and stored in a cool, dark place to be re-opened next year. And even if your garden produces more food than you or your family can consume, you could either donate or preserve the food for later use. The last thing you want is your edibles going to waste. If you're new to gardening and aren't sure what to expect, start small. A single tomato or pepper plant will still save you money at the grocery store, and the experience of growing just one or two plants is valuable when deciding how much to expand your garden in future seasons.
Lastly, and most importantly, decide what seems right to you about record keeping. Some people choose to be very meticulous and chart the variety of vegetable or fruit they plant each year, where they planted it, when it was planted, its estimated harvest date (available on the back of every seed packet) and its actual yield. Some choose the low maintenance route and simply noting what types of plants they grew and where (important for crop rotation, which is imperative for crop disease prevention in certain plants, such as tomatoes). Biting off more than you can chew with planning and record keeping can be more stress than you want to introduce to a hobby. On the flipside, good charting can help make your garden the best it can be year after year.
All About Watermelons, Burpee.com
How Much Sun Do the Plants in My Garden Really Need?, Gardening.About.com
Vegetable Garden Layout, HumeSeeds.com
Garden Tips, MVSeeds.com