Every user on a PS3 already has a password. When you start a new user account, part of the
set-up procedure is putting in a password.
If you have multiple user accounts for different people, and you want to keep others from using other accounts, first be sure that you do not have that account set for
Automatic Login. After that, go to the "Friends" menu in the XMB and highlight your online PSN name, then press the triangle button.
Be sure to turn off the Auto Sign-In option
(it will be shaded if it's off). Then Sign out. Now go to the PlayStation Network menu in the XMB and sign in.
Now you will see a pop-up window showing your sign-in ID (email address) and password ******.
If you uncheck the box that says "Save Password", then no one can access that account without first putting in the password.