Starting your tomato seeds indoors can be a great way to get a jump on the growing season and to have young plants ready to make the transition outdoors sometime after the last frost. Starting them yourself is also considerably cheaper than buying tomato plants from a nursery later in the season.
Begin your indoor planting about six to eight weeks before the expected date of the last frost in your region (see resources).
Fill a seed tray with seed-starting mix, and water lightly.
Make shallow rows by running your finger or the end of a pencil over the surface of the seed-starter mix.
Drop tomato seeds into your rows about a half-inch apart, and cover them with a quarter-inch of seed-starter mix.
Label your trays so that you can remember which varieties are planted where. One of the most enjoyable aspects of growing tomatoes is the number of different varieties you can enjoy in a season.
Find a warm spot where your trays can remain undisturbed. A constant temperature in the high 70s is best.
Make sure the trays have plenty of light once the plants begin to germinate---a sunny window is ideal. You can also supplement natural light with artificial light. Tomatoes will grow "leggy" without sufficient light, so ensure the seedlings get 12 to 16 hours of light each day.
Water your trays regularly throughout the germination and early-growth phases. Your seedlings shouldn't sit in water, but neither should they dry out. A consistently moist soil is best.
Prepare enough pots to transplant your seedlings. You'll need a set of 4-inch-diameter pots filled with potting mix.
Transplant the seedlings after about 30 days, once they have grown a couple of sets of leaves. To transplant, make a hole in the potting mix in the new container. Use a teaspoon to scoop the root ball with the soil around it out of the seed tray, and then gently put the root ball into the hole in the 4-inch pot, filling the area around it with potting mix. Water well. Discard any seedlings that have failed to thrive.
Keep the pots with their young tomato plants indoors until outdoor nighttime temperatures are at least 12.8 degrees C. Transition them to outdoor living by bringing them out in the daytime for a few more hours each day.
Plant them in the garden, spacing them about 2 feet apart, or place them in outdoor containers. As tomatoes grow, ensure they have plenty of support from a cage or stakes.