Tomatoes are the most rewarding and easiest vegetable to grow as transplants. My advice: Grow your own transplants because only the best varieties yield the best-tasting tomatoes - and you rarely find the best varieties for sale as transplants.
One mistake people commonly make in raising their own tomato plants is sowing seed too soon indoors. Not only do such plants have to be fussed with for a longer time, but they also end up with spindly stems and cramped roots. Plant seeds indoors just six weeks or so before you normally plant tomatoes outdoors.
Besides seeds, you need potting soil and any container a couple of inches deep with drainage holes. Plant the seeds a quarter-inch deep, water, cover the container to hold in moisture, and place it in a warm location. An ideal temperature is 75 F, but don't fret if the temperature wavers even 10 degrees from this. As I said, tomatoes are easy to grow.
Peek at the container every day. As soon as seedlings poke through the soil, they need light and air, so uncover the container and then move it to a sunny windowsill or to within a few inches of a fluorescent light.
In a few days, new leaves will unfold. If you sowed seeds in individual, small containers, thin out all but one seedling per container. If you sowed the seeds in one larger container, you can gently lift each seedling by its leaves and drop it into a waiting hole, prepared in potting soil in a small container.
This latter method makes more economical use of space, initially, and makes stockier plants later because you can bury each seedling in its new hole right up to its leaves.
For robust growth, now give the transplants the brightest possible light without letting them get overheated, all the while keeping the potting soil moist but never sodden. Sixty to 70 F is an ideal temperature. Watch out for yellowing of the lower leaves; if this occurs, apply a water-soluble fertilizer.
A week before you're going to plant your tomatoes in the garden, start acclimating them to the great outdoors. Begin by moving the plants outside for a portion of each day to a sheltered spot, then gradually leave them out longer and in a more exposed location. After a week or so of acclimating, they are ready to be planted outdoors. If you did everything just right, the plants should be stocky, a rich, green color, and about ready to flower.
Among the best-tasting tomatoes are the varieties Sungold, Brandywine, San Marzano (for sauce), Carbon, Belgian Giant and Anna Russian.