Should I Keep this turtle?

I found the cutest little turtle in my yard he is like less than a pound it look like he is a baby there a lake by my house but i dont want to put him there and my mom doesnt know i have him in my room in my old turtle tank its small and under my bed im gonna tell her she is gonna freak but if i keep him what do turtles eat? and do they need playmates? and how do i know if its girl or boy?