The stems of seedlings are quite pliable and will grow towards the light source. This enables the plant to collect more energy by having as much leaf surface area in contact with direct light as possible. In nature, plants will often follow the sun across the sky from east to west. Make sure you have your lights directly above your plants. If you're using florescent tubes, remember that the light is always strongest directly in the middle of the tube. Plants placed on the outside edge of the tubes, where the light is weakest, will sometimes grow toward the stronger light in the center of the tube.
When to start adding nutrients to your soil depends on the type of soil used. Most soils have enough nutrients to promote healthy plants for at least a couple of weeks. After this time, nutrients will need to be added, but go easy in the beginning. It's easy to burn young plants.