If you mean a jumper to clear the CMOS password, which comes up on a text screen on it's own, in the middle of the screen (usually), when initially starting the system up - then it's usually nearish to the battery. The motherboard manual will show where - not all motherboards are the same. Otherwise you can take the battery out - leave it out for 2-3 hrs.
I find it difficult to beleive nobody can remember a password that would have to be input every time the system is switched on!
If you're talking about a password to get into Windows, that's a very different kettle of fish.