The box turtle site is good.
Lots of vegetables, some furit (10% of diet) ; protien in measure, in the form of insects and earthworms. Some vegetables are not recommended, such as spinach. I think the Box Turtle Site has a list of good and not so good stuff. I add calcium in the form of powdered cuttle bone once a week. Mealworms are fattening and addictive - don't introduce them. No raw meat, no beef or pork. Boiled skinless chicken breast can be used occasionally.
Tough love may be required to get him/her to eat right if they haven't been fed properly. They are a lot like kids in that they lock in on certain foods, like fruit or meat worms and ignore everything else. If that's the case you can mix in their favorite food with good stuff and gradually reduce the favorite food over a period of months.
Box turtles do better outdoors, but if your climate is getting cold now, indoor digs are better until you can move them outside in the spring. They need to experience about 14 hours/day of uvb in the form of sun or a lamp, othewise they may exhibit hibernation behaviors. Make sure they have a few options for to get into the shade (micro climates). A shallow water dish should be provided for drinking and soaking. Clean it daily cause they like to poo in their water