It all depends on what kind of turtle it is. Water turtles, I can't help you out too much, except that they still need someplace to bask and I would recommend getting them live food such as goldfish once in a while, too.
Land turtles--Box turtles, to be more specific--there I can help you; I am presently keeping a baby female box turtle, can't be more than 2 inches long. I've had a lot of trouble taking care of her in that she won't eat. You could not imagine how difficult it is to get her to eat. She's in a fify gallon terrarium with all the sides plastered with green paper, otherwise she'll feel exposed and then she DEFINITELY won't eat. She has at least 3 inches of peat moss/potting soil substrate, a large, shallow waterbowl, live plants, plenty of hiding spots, and an owner who is constantly going through great pains to get her to eat... I think the main reason it's so difficult is because she was wild caught (I hope to release her come spring) by my friend. NEVER get wild box turtles as pets!!! Try to adopt one from a Box turtle rescue site or something.
Anyway, there is one thing that I cannot emphasize enough no matter what kind of turtle you get: Turtles, like most reptiles, NEED UVB LIGHTING. Without UVB (and plenty of calcium, might I add) turtles cannot turn calcium into bone. This results in metabolic bone disease and a small or deformed shell, easily shattered bones, soft or short jaws, deformities in the bone structure, and, ultimately, death.