When they're young turtles are almost strict carnivores. Don't worry, they're supposed to be. As they age they will begin to accept more and more vegetable matyter, but the majority of their diet will still be meat.
Turtles need an incredably varied diet. Fish, crayfish, earthworms, bloodworms, krill, squid, daphnia, brine shrimp, crickets, locusts, mealworms, superworms, waxworms, butterworms, cutworms, leeches, pinkies, cockroaches, and food sticks are good options to include in the meat department. As far as veggies go, dandelions, collards, bok choy, and romaine are best. Its more the greens that you're going for than actual vegetables when it comes to aquatic turtles, although the occasional veggie or banana piece will be good too.
Make sure you've got the right temperatures though, and be sure you have the right sort of UVB light and that yoiu change it often enough. Without the UVB light your turtles will die.