What should a turtle NOT eat?

It depends on the species but over all turtles shouldn't eat dairy or milk products, rocks (i know it sounds crazy but some turtles will eat rocks and be okay but others will have an impaction and die), raw or cooked shrimp (unless sold as a turtle food in a pet shop), and potato chips and human food in general.

You CAN feed turtles hard boiled eggs (just keep them from gorging them selves (females and babies love them sooo much that they often stuff them selves to the puking point, males just think their okay), tiny amounts of raw ground beef, cooked chicken, and soggy dog food, earth or blood worms, crickets, snails, tadpoles, and several kinds of aquatic plants. Turtle pellets are a very good option also.