What are the special requirements needed to care for a turtle?

I'm really concerned because my younger sister brought home a small turtle she found at a beach she went to. And yes, I'm mad that she even brought him here, taking him away from his natural habitat. She wont even care for it soooo that's where I come in. I DO believe it's a Map turtle but I'm not entirely sure. I'd like to know EVERYTHING I need to know about how to properly care for this turtle; tank size, food, temperature, etc. EVERYTHING. I've already grown attached to him. And how much would everything in the end cost? He's fairly small.