How do I take care of this baby turtle?!?

I have recently aquired a baby turtle from a little local fair. I really was not looking for it, but I felt so sorry for the poor thing in an aquarium with just water and about 50 other baby turtles. I know little about caring for it, or even what it needs. I've gotten an aquarium for it already, with a heating lamp, all water with a plank that comes from the bottom to the top, which holds via suction cup. I put the heating lamp above the dry area (the plank) and littered the bottom with fish gravel. I also threw a couple rocks in the tank. When I first got him in a tiny plastic cage, I threw him some fresh lettuce, but i'm not sure if he's touched it at all. I got some stuff from petco for the water, which helps clean it apperantly, and I also bought baby turtle food, but he doesnt seem to be eating it. He/she/it seems to be very shy, and often hides when I come into the room. What can I do to get this thing to eat?! Also, is there anything else I can give it to make its place better?