Starter, alternator and wires

I had to change the starter so I did then the battery light came on. So I disconnected the positive wire and the car shut off. So I went and got a used alternator and put it on the car and it still had the battery light on. So I took the alternator back off and had it tested and it came back good. Well when I put it back on I saw a wire broke by the starter. It is one of the red one's and I do not know where it goes. And it will not charge the alternator. The ground wire that hooks up to the tray the battery site's on rusted and broke off. So how do I ground it back to the frame? The relay we got it came off too. No clue where it goes either. Some one please help!