Why does my car starter stay on?

i just got a 91 mustang 5.0 and a few days ago. sometimes it starts fine but sometimes the starter stays on when after its running even when i turn the key back or off. so i unhook the positive from the battery so it stops. and sometimes when i hook up the battery back up, the feul pump kicks on and then it trys to start itself when the key isn't even in it. so what could it be? but then somtimes it works just fine.