Let me guess...this isn't your computer, is it? Why should I help you? You'll get caught, that's why, and the owner can't find me, so he will kick your tail instead. Oh, but if it's a school computer, they will probably just roll their eyes and wag a finger in your face, because they have no real punishment options.
You absolutely cannot ferret out anyone's password on any UNIX system. You can change the password, but... did I already say this?... you'll get caught, because you can't change it back.
Boot to single user mode. Type "/sbin/mount -uw /"
Press Return. (Nothing appears to happens, no problem. Also, you will see # again. That's fine; ignore it.)
Type "rm /var/db/.applesetupdone" (The file actually has some upper-case letters, but the normal format of Mac OS doesn't care.)
Press Return. (Now stuff appears to happen. You will finally get past the "Welcome" in a dozen languages and then you can create a new admin user. Open System Preferences > Accounts. Select the other admin account. Click the "Change password" button.