Q: how do I log out a guest user?

Apple menu > Logout Guest

Reset Password

OS X 10.7 Lion /10.8 MountainLion

Follow the instructions in the first and the third boxes.

http://pondini.org/OSX/Password.html

Note

Keychain

http://support.apple.com/kb/TS1544

Green box.

Click in the box under "Select the user account".

Select your administrator account.

A user can be an "Administrator".

OS  X keeps all passwords in an application/utility called Keychain Access

for easy management of various passwords.

Best.