Now that your tomato plants have found their home outdoors and they've started to grow a bit, you'll need to support the plants so that they grow upwards. Tomato plants naturally vine, and if left to their own devices, they will crawl along the ground where they will be more susceptible to rotting, bugs, and disease. So, it's best that you create a support system to encourage upward growth (doesn't this sound like it will result in a happy tomato family:). As the plant grows, you will want to "train" the main vine up whatever support system you choose. Training the plant simply means gently moving the main stem against the cage/trellis/string [read about these methods below] as the plant grows.
There are a few apparatuses you can use to support the plant:
Metal-wired cages can be purchased at most gardening supply stores. This is an easy option since you don't have to construct anything; just sink the metal tines in the ground around your plant. The cages are probably the least ideal method of supporting your plants. They are not quite tall enough, and they will not support the weight of some of these larger tomato plants. However, I think they are sufficient for supporting cherry tomato plants or other smaller varieties.
I also read about creating your own cages with 5 foot tall "gardening fence". Basically you create a tube out of the fencing material. This would probably be the most ideal option, but it requires buying extra material and a little more labor. If my tomatoes completely flop this year, I may look into it for next year.
A few years ago, my husband made a few smaller trellises by using thin lumber and nails. He cut two pieces the desired height we wanted the trellis (ideally, five to six feet high) and a couple cross beams to hold them together. Then, he simply nailed the pieces together. Stick the ends in the ground overtop the tomato plant and train your tomato vine up the trellis as it grows. If you want the trellis to last longer, you might consider screwing the wood together instead of nailing. These trellises may only last a few years since the wood has already started to rot a bit.
Gridded Stakes (Stakes with Twine or Rope)
I learned about this method last year from a friend that has wonderful success with her tomatoes. So, I've decided to give it a try this year. Use approximately 5 feet tall stakes and edge the tomato plant bed with them. I think my friend uses old bed posts; my husband cut some scrap lumber. You should have one in each corner, and one about every 12 - 18 inches apart. Insert them in the ground and make sure they are stable. Then, use a sturdy string or twine and wrap it around the stakes about 6 inches from the ground making a grid pattern. I read that fishing line or piano string is not ideal as they may cut into the stems of the plant. Repeat this grid pattern every 12 inches up the stake. I would recommend waiting to add the next string grid layer until the plants have reached that heigh; it will be easier to train the vine up the grid.
Whatever method you choose to support your tomato plants, make sure you train the main vine of the plant to grow up and around the cage, trellis, etc. This will provide the proper support the plant needs, which will [hopefully] help yield the most delicious fruit!
Check out my other posts on My Tomatoes' Journey.