There are a number of diseases that affect tomatoes, but plant scientists have developed resistant cultivars. Choose a good location. This may be the hardest task. Tomatoes need soil with a pH of 6.0-7.0 and require full sun. Tomatoes and corn don't make good companions because the tomato fruitworm is identical with the corn earworm, and we are told that tomatoes make potatoes more susceptible to potato blight.
Tomatoes do quite well with chives, onions, carrots, garlic, and marigolds.
Till the soil. The plant will need support as it grows. If you decide to use stakes it is wise to drive them into the ground at the appropriate points before you set the plant. Dig the hole, and set the plant in so that the top four inches remain above the ground. The goal is to develop a good root system, which will ultimately produce more tomatoes.