Join this forum, couldn't be more helpfull.
They can be nasty if they think you don't care or just bought on a whim, but if you explain the situation & you want to learn, they will help no end.
http://www.reptileforums.co.uk/forums/shelled-turtles-tortoise/I know nothing of turtles really.
As a kid we found a red eared slider (terrapin), we fed him raw mince, liver, dried shrimp ect.
They may take frozen bloodworm ect, food items should be no bigger than the gap between their eyes. A varied diet is best, sticks as a staple maybe with other foods added.
These people on the forum keep them or similar species so they speak from experience.
http://www.reptileforums.co.uk/forums/shelled-care-sheets/166307-aquatic-turtles-terrapins.htmlReading the care sheet, they take greens/veg.
Take notice of the calcium supplements, reptiles in captivity tend to lack calcium & it can kill (MBD) metabolic bone disease.
Vetark products are good, but keepers with experience can point you in the right direction, as I only know lizard needs.
I've probably made you think it's hard to keep reps, but it's not, get the basic right & they are easy, get it wrong, then it becomes hard.