It really depends on what type of turtle it is. First off, is it a tortoise or a turtle? Is there any way you could get a picture of it? If it is a tortoise, it will most likely only eat greens (romaine lettuce, spinache, dandelions, mustard and collard greens, hibiscus leaves and flowers). If it receives too much protein in its diet (from insects and animal fat) his shell willl develop prolapses and "bumps". An overload of protein is just as bad as an "underload" of it or any other vitamin. Just keep that in mind.
BUT, if it is a turtle it can eat many different things including minnows, goldfish (but I wouldn't really recommend those as a primary item of diet), shrimp as mentioned above, and greens. So you see, it is important to know exactly what you found. Where do you live? That may help to determine what you found.
Let me know.

Jennifer