The Short Answer:
Depending on the environment that the box turtle came from, their diet will be a little different. In the wild box turtles will often eat snails, spiders, earthworms, caterpillars (careful as some are poisonous), grasshoppers, fallen fruit, berries, flowers, and grass. Their diet greatly varies by what is in their natural environment. Before deciding on something to feed your turtle, you should find out what it has eaten in the past, or what type of environment your subspecies lives in.
There are man commercial box turtle foods that can give your turtle the vitamins, minerals, and proteins it needs to survive, however these foods usually cater to every species of box turtle, making it lack some types of food, and have too much of other types for a specific type of box turtle. It is a good idea to use these commercial foods, look at the ingredients, and add some more foods to its diet depending on the species you have.
What Food Should Look Like:
A good food should have roughly 50% of the food containing foods like slugs, earthworms, dirt bugs, and other small insects. Although live bigs are better for turtles, some foods may have dried bugs in them.
The other 20% of their food should be a mix of things like mushrooms (non toxic) and berries.