Put tomato plants in the sunniest window in your home. Rotate the containers once each day so each side of the tomato plant gets enough sun.
Set the tomato plants under grow lights or fluorescent lights if no window in your house provides enough sun. Hang the lights about 2 inches above the tomato plants. Keep the lights on for 12 hours a day if your tomato plants are overwintered. If the plants are seedlings, they need 16 hours of light per day.
Set a fan in front of the tomato plants and turn it on to the lowest setting. Let the fan blow on the tomato plants; this helps them develop strong roots and stems.