First, location. You need a place that gets at least 6 hours of daylight, and water can be brought to it, leaving it not too wet and not too dry. Have both the soil and water tested. Some areas have had so much insecticide sprayed on it, that it is now in the water. Have the soil tested for things like lead. Check with your local extension office about the testing. Once upon a time it was about $7 a vial in Sedgwick county.
Also have your soil tested to see where you are on ph levels. Do a little research on what it is you plan on growing, and what it's soil requirements are. Start building up your soil by adding hummus, lime, manure, and so forth. Attempting to create that perfect ph. Also look for earthworms. The presence of them means a healthy soil. I would till everything under at this point. You will want to do this a couple of times, churning up as much as the weed and grass seeds as possible.
Plan out your garden on paper, from plot size, to what, where to put plants, and irrigation. Know your area, and it's climate, these will be a factor in what you can grow.
There you go, on paper it is simple. But there is a lot that goes into your garden once you know what obstacles you are facing. If done correctly, expect $15 worth of produce for every 1 hour you spend working in your garden.