How can I fix my iPhone?

So I found an iPhone which is a 32 gig with a white back (guessing it's a 3gs) and I want to activate it with AT&T but it's stuck at a screen saying slide for emergency. And the phone says sim is locked with puk. When I connect it to iPhone it says cannot read sim, and if I put an AT&T sim it says sim not supported. How can I activate it? Or make it work?