Young turtles of that size should be fed as you would an adult, just keep in mind that they can still be very timid about being out in the open. You should start introducing other foods, because it will need more variety in its diet than you are currently providing.
Different folks use different substrates: whatever you choose should retain some moisture so that the ambient humidity is maintained at high levels. Garden soil mixed with peatmoss, Bed-a-Beast mixed with soil, & peatmoss alone are a few that are mentioned...
As for water dish: use anything that your turtle can get in and out of easily, and is portable enough for you to remove & clean with ease.
I'm including a link to Tess Cook's website about box turtle care that has alot of great information (particularly about diet) that should get you started.
Make sure that you are providing adequate UVB light as well: ordinary incandescent bulbs aren't full spectrum.
Stephanie
Link