It all depends which turtle you have, they require different things
Before buying one, see which you want and do alot of research so you have everything set up perfectly before getting it

Depening on what kind of, and how big the turtle is...It's probably a 3 toed box, thats the most common, or a snapper? The tank for the turtle should have room along the bottom to fit atleast 6 more of them in (that's how i was taught to size a tank for an amphibian) Get some dirt, this stinks, but put it on a cookie sheet and bake it on 350 for atleat half an hour to sterilize it, add water to make the mudd and let it settle in the bottom of the aquariam, you can mix some sand in with it too if you like, but a small amount, and rocks and such too. make sure they have enough water to submerge themselves in, and a piece of drift wood, or a rock with some plants around it to come out and bask on. If you have a heating lamp, only turn it on when he comes out, because he needs warmed their cold blooded animals.
They like to eat small frogs again depending on the size of your turtle, crickets, meal worms, some types of plants, crawfish (make sure you rip their claws off first...it sounds mean but they long them) basically anything you'd find in a pond ...you can also look some of this up on www.google.com and find a lot of info too