Cute Little Fella
I agree with Jess that if it was taken from the wild it should be left in the wild. In PA it's illegal but I realize that doesn't apply to all US states.
If you still insist on keeping the little guy, here's some food for thought:
It kinda looks like what we call around here a Green Slider. As a kid I could buy a chameleon(anole) or one of these "Green Sliders for about $2 at the county fair. They sold these for years along the northeastern part of the states until someone started to get salmonella from handling these little guys. I think there was a little more to it than that but anyways they(the government) put a band on the sales of these creatures. They said pet shops as well as fair vendors were no longer allowed to sell them anywhere in the US until they were at least 5 inches.
I also had a Map turtle that also looked very much like this turtle when it was only 2" long. I had this turtle for only 3yrs and it went from 3 to 11". I started him in a 10 gal tank with a baby red eared slider until they were both about 4". From there they went to a 20gal tnk. When the red eared was 5" and the map was 7", I had to separate them both into their own 40 gal. home. The map turtle was dominating the other and the red eared wasn't getting the proper nutrition.
The Map turtle was recently given away to someone who does reptile shows in our local schools. I still have the red eared.
From what I heard, the growth of a slider and the map are about the same. As far as feeding them, when they're young, you can stick to the pellets. As they get bigger (3 - 4"), both types of turtles will eat about the same foods such as a good grade of calcium enriched floating turtle pellets/sticks, worms, collard greens and occasionally live small fish as a special treat. A cuttle bone can be floating in the tank for them to nibble on for their calcium. UV lighting is also important. But be prepare because water turtles are very messy and require a great deal of care. A higher end submergibal pump/filter should be installed in the tank like a Fluval Plus 3. Every 2 weeks their water should be changed and filters washed in it's own water. It sounds like a lot of work but if you like turtles as much as I do it's worth the effort. - Chicojo