Set up a place where he can swim, in a terrarium or tank, but also make sure it has a place to get out and sun itself, and dry out. A lamp for warmth is also a good thing. You can try bits of lettuce, or greens, or even bits of meat for now, but be prepared to take them out of the water if it doesn't get eaten, you don't want rotting food in the water. Don't panic, it can go a day without eating, and you can get turtle food at a store when you get a chance. Making sure it has clean water, a place to dry out and a place to hide if it is scared are most important right now.

Once we know what breed of turtle it is, and see a picture, we can help more!