I know you're thinking, "Dig a hole, put it in the ground, water, done." While that method might work well for you, I have a few tips and tricks that can help to provide you with larger, healthier tomato plants. These are all lessons I learned while working at The Plant Place when I first started attending West Georgia. I use these steps to this day. Please remember that tomatoes love sun but do not like major heat. If your yard gets exceptionally hot in July and August, you might want to find a location that will allow your tomato a little shade during the afternoon hours.
1) If you are planting your tomato directly into the ground, set the tomato down where you'd like to plant it. Using a trowel draw a circle on the ground around the plant that encompasses the width of the widest branches on the plant. You will need to dig up and break up all of the dirt inside this circle. The tomato's roots will need for the soil to be loose for those first few weeks of growth. Generally, a plant's roots spread out as far as it's widest leaves. The roots will in many ways mirror the top of the plant. (This rule does not apply to taproots, amongst others, for you smarty pants out there waiting to let me know). You'd like for the plant to get as much root growth as possible before it begins to bloom and bear fruit. Loosening the soil will allow the tomato's roots to easily unfurl from the confines of the pot shape. Now is also the best time for you to amend the soil with compost if you are choosing to do so. If you are planting your tomato in a planter, you will need to make sure that you have a large pot with nicely loosened soil in it. You can choose to loosen the roots on the plant as well before you place it in the ground. I will only do so if I think that the plant has become "root-bound" from being in too small a pot for too long.
2) When digging the hole for your tomato, it needs to be deep enough to cover the first set of leaves on the plant. Before you place the tomato in the hole, you will need to pinch the first set of leaves off of the plant. Why you ask? The tomato will develop an additional set of roots from the vacancy left by the leaves you pinched off. A good root system = a healthy plant = a great crop.
3) Put your tomato cage on NOW! You make think your plant is too small to need a cage. However, by the time it needs the cage, it will be damn near impossible to get the cage on the plant without damaging all the beautiful leaves and flowers step 2 helped you to attain. All tomatoes need some kind of support system. Make sure you know what you are growing. Bush plants can use a regular tomato cage, while vine plants will likely need a trellis or fence for support. This is also the time to begin stockpiling old panty hose from your grandmother. The nylon legs work well as strips for tying the tomato plant in place as it grows larger and larger without damaging the branches like string or yarn would.
4) Mulch, mulch, mulch. Mulching will keep the weeds at bay and helps the soil around your tomato to retain moisture (esp. during the Dog Days of Summer). There are a variety of mulches that would work beautifully: Pine straw, hay, bark, even river rock! One of the most economical "mulches" you can use is your weekly newspaper. Simply cut notches into the paper so that it can lay flat against the ground and hug the base of the tomato plant. (Regular newsprint is my recommendation, not the glossy sales papers). If you use the paper, you will likely need a few rocks to help keep it weighed down until after a couple of waterings.
5) Water!!! When you first plant the tomato, it will need to be soaked well for the first week or so. However, after that first week, your tomato will only need to be watered when it is dry. Tomatoes can be very sensitive to over-watering and develop root rot. IF you stick your finger an inch down in the soil, and it is still dry, you need to water. Otherwise, wait a day and check it again. If your tomato is wilting and has brown crunchy leaves on it, it needs to be watered more often. If your tomato is wilting and has yellowing leaves, it is being over-watered and likely developing root rot.
6) Fertilize your maters. Always avoid getting fertilizer on the plant's leaves. If you are choosing to grow "un-organic" tomatoes, go right ahead and choose a tomato fertilizer at the store and go to town. However, if you are choosing to go "green," fertilizing your tomato can seem like a challenge. Stores do sell pelleted "time-release" organic fertilizers that you can sprinkle on once at the beginning of the season. Or you can opt to invade your kitchen for other ways to boost you plant's potential. Tomatoes enjoy crushed eggshells. A little Epsom salts mixed in with plant's water every two to three weeks is another option. Drink tea or coffee? Make the tomatoes a pitcher full with the left over leaves or grounds and water away. You might choose to do a soil test, which can be done with the help of your local ag center, before planting to see what your soil needs before planting or fertilizing. I have never done a soil test and only fertilize my tomatoes intermittently. I rarely remember to do it on schedule. I do aerate the soil well and mix in fresh compost (from my compost bin) every year.
By the way, all of the natural fertilizers listed above can be used on other plants.