Put them in the sun during the day and bring them in at night until all danger of frost has passed. Check out the estimated last frost date in your area.
Tomatoes like lots of light and a constant mild temperature.
They hate cold, windy, wet weather (a bit like an English summer ... just joking!), too much watering and poor light levels. As the plants grow transplant them into bigger pots.
Some gardeners use grow lights to supplement the daylight hours or extend them. This is a good idea if you want to sow early and have tomatoes early, but it can be a bit expensive in energy bills.