Once you've got your tiny tomato plants sprouted from seed, it's time to move them into the light. You'll want to put your seedlings in the strongest light you have available. For many gardeners this means a sunny window, which will work. Even better is a strong florescent light or grow light that can be placed just a few inches above the seedlings (an adjustable grow light system can help a lot in this). The closer and more powerful the light is, the less the light will be diffused, which will mean your tomato plants will grow stockier and not get leggy (tall and spindly). If you don't have powerful enough light and the plants do get leggy, it's not the end of the world. Tomato plants have the interesting ability to grow roots along the main stem, which means that even a plant that's too tall to be stable when you transplant it can just be buried deeper.
When placing your tomatoes in the light, keep in mind that you don't want the light so close that heat from it burns the plants, and it needs to give room for air to circulate over the seedlings (keep them protected from chilly drafts, though). Around 14-16 hours of this light a day is optimal. Some gardeners recommend longer, as much as 20 or even 24 hours a day of strong light. I've never found there to be any improvement with more than 16 hours of light, and in some cases it can even be harmful. Plus, conserving electricity is a concern for pretty much everyone these days. If you're growing your seedlings with a directional light source (a window, a smaller light or one that isn't directly above the plants, etc), be sure to change the orientation of your seedlings to the light. When I'm growing seedlings in a window, I try to turn them around once a day, so that they don't bend over trying to grow toward the light.