How do I start tomatoes from seed?
The right time to plant tomato seed is 10 to 12 weeks prior to your average last frost date.
Tomato seedlings are susceptible to damping off, so a warm area, soiless mixes amended with a small amount of bone meal and a fresh new florescent light bulb set close to the tops of plants is in order. By close I mean no more than six inches from the growing tip of the plant.
The debate of 16 hours of light vs. 24 constant light per day may never be satisfactorily won by one side or the other. However cold soil leads to damping off, so when the lights are off the seedlings still need to be warm (75 to 85F). At home that may be on top of your refrigerator, at night.
Plan on potting up at least once from what ever you use to start seedlings in. In fact twice (i.e. from a six pack to a 4" pot, to a 6" pot) every time the roots start to wind around in their pot.
Never ever use a loess soil based "potting soil" to start tomatoes, the soil particle size is too small to permit good gas exchange in the roots. Use a soiless mix or homemade equivalents, made of 2 part peat moss, 2 part finished compost, and 1 part vermiculite or pearlite, and a dash of bone meal. Pure peat--peat pots work for some people I find them more than averagely susceptible to damping-off problems. Mixes containing milled sphagnum have a natural antifungal effect, resulting in less damping-off.
Hardening seedlings is an art. The short of this subsection is to gradually let your plants become accustomed to full sunlight and wind, find a very protected area and start on days above 60F with wind protection. Expect it to take 10 to 14 days of hardening off.
From 2 weeks before your last average frost date and two weeks after you will want to plant to field. If you must strive for the first ripe tomatoes on your block and you do not have a very protected microclimate in your garden you must look to things like wall'o'water towers or some kind of temporary cold frame to erect around your seedlings. If terms like 'micro-climate' are greek to you, plan for a later planting date rather than an earlier one.
You should be planting in a well drained bed that has been prepared in advance of planting. Wet cold gooey soil means that not much growing is going to happen. Corn and tomatoes both like soil that is warmed to 60F.
These are particularly helpfull for indeterminate plants which can weight in excess of 100 pounds each. I recommend the stoutest and tallest trellis you can afford to build.