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Select a pot, any kind will do. I like plastic, because it's light and comes in a full array of colors. You will need a pot 5 to 10 gallon in size. You want it to be as deep as it is round (or square). It should be at least 12" in diameter for a single plant, 14" in diameter for two plants. You need to select a pot with drainage holes in the bottom. A pot that comes with it's own bottom saucer to catch that drainage would be perfect.
Next, place the pot in a location with adequate sun. You can grow tomatoes on a porch, balcony, deck or yard. Put a little loose gravel in the bottom of the pot to provide better drainage of the pot. This also helps keep the soil in the pot, instead of running out the bottom holes.
Fill the pot with a good potting mix (not garden dirt). A potting mix allows the roots to grow properly and affords better drainage. Soil is too dense. A regular sized bag (40#) should be enough for a 5 gallon pot. You will want to add some kind of calcium source.
[NOTE: I have had lots of luck with Miracle Grow Organic potting "soil". Generally mix is considered better, but this was good for me. I would even recommend mixing the two together,]
Since you are using a potting mix, there is not a source of calcium. A lack of calcium can cause "blossom end rot". A good type of calcium for a container is "bone meal". It supplies N 3, P 2, K .5 & 24% calcium. This is a great fertilizer to kick off the tomato plants health. Use about 2 big handfuls for a 5-10 gallon container.
I had an online friend ask about the use of dolomite to correct any PH changes due to use of fertilizers. PH is far less important in containers, than their garden counterparts. Generally, you are starting with a PH balanced potting mix. Fertilizers will not usually have a noticable effect on the usauble calcium unless you use too much nitrogen and/or lock up the calcium. That is why I prefer to use something like a kelp or seaweed mix, if I need additional fertilizer. You don't get those high numbers like 20-20-20. If PH is of an over concern for you, substitute hydrated limestone for the bone meal. Use about 2 cups or so for a 5-10 gallong container. Dolomite could also be used, but it is about 50/50 magnesium and calcium carbonate. Calcium cabonate is not the most effective calcium for a container and you may not need the magnesium.
You need not fertilize much, after planting. Once a month will be fine. Do not use the high strength "Miracle Grow". A tomato only needs about a 3-5-4 fertilizer. Divide your Miracle grow strength by about 5 or use a liquid kelp or Neptune's Harvest with similar numbers to 3-5-4. When you plant your plant, give it a little liquid kelp, seaweed,fish emulsion or earth juice. I prefer earth juice, because it's not too high in nitrogen.
You don't want to force a tomato plant's upward growth before it developes roots to feed it.
This will help feed the plant, until the bone meal begins to break down.
The next step to consider is, what kind of tomato do you want. I find that determinate tomatoes work best. This kind of plant stops it's growth at a certain point, making it more suitable for a container. All the fruit on a determinate plant matures on it's bush like structure.This is in contrast to indeterminate which proceeds upward, unchecked; requiring growth for more fruiting. If you choose to try an indeterminate tomato, then I recommend using a 10 gallon container for the one plant and adding a little bone meal or organic fertilizer into the potting mix. This will help to feed the larger indeterminate plant, far better than just a liquid feeding.
At a nursery, you will probably find few choices, such as Celebrity, Patio, Better Bush and the Mountain Series (Mt. Gold, Mt. Pride, Mt Supreme, etc.) There is a new semi-determinate cherry type called, Sweet Baby Girl Hybrid; which grows excellent in a container. If you have the time and inclination, to grow you own plants. then check out my TOMATO GARDEN TIPS page.
Choices are unlimited when growing your own from seed.
If you have chosen to try an indeterminate, then you have a lot of choices, such as; Better Boy, lemon Boy, Big Girl, Big Boy, etc. etc. Once you have the location, the pot, the soil and the plant; guess what? It's time to plant.
Remove the tomato plant from the six-pack or pot and carefully place into the potting mix. You should have made a hole to accommodate the roots and part (1/3rd) of the plant. Placing 1/3rd of the stem into the soil promote root growth, which you will need. Now water the plant until all the potting mix is wet. Water carefully, because you can flood out the seedling and have to replant.
Now place a small stake or tomato cage into or next to the pot. I use a fairly large tomato cage for a pot, but then I use determinate plants. An indeterminate tomato will out grow a cage, so use a large tomato stake. If you have chosen a balcony or patio etc, to place your pot, then a stake is almost useless, so use the biggest cage you can find and secure it to the pot. I use a tape or cord the same color as the pot. You could place the pot near the edge of the structure and have a lattice support the plant.
Keep the pot moist while plant is getting adapted, but not too wet, that you will stop root growth. This is usually not a problem in a pot, that is sitting in the sun; anyway. I water almost every day. Pots require more water, then a garden does.
Now all you have to do is wait for those tomatoes.
BTW: Don't trim off side shoots or you will lose tomatoes! Never trim side shoots on bush or determinates. because what you see is all you get!