I am looged on currently and in system preferences. However, I can't remember my old password so that I can change my password. Thus, I can't log out until I fix this problem.
MacBook Pro, OS X Mountain Lion (10.8.5)
Resetting User Password in Lion, Mountain Lion & Mavericks
Boot into your Recovery Partition by holding down the Command and R keys whilst booting.
When the Recovery screen appears, go to the Utilities menu and open Terminal.
In Terminal, type:
resetpassword...a small app will open and allow you to choose the user for whom you want to change the password. If you are the admin, be certain that you choose your admin user. Type in your new password twice and give yourself a good hint. Click OK (or Done or whatever) and reboot, enjoying the use of your new, hopefully memorable, password.
Good luck,
Clinton
Logged on, but forgot my password so I can't log out