Lang Wongman-
Check System preferences, the "Accounts" pane. On the "Security" tab, there should be a user listed who can "administer this computer". If it is not your regular account, you may need to log in as whatever account is listed as having the privileges to administer this computer.
If you need to reset the password of that account, you need to authenticate, by clicking on the lock and entering your password. Unfortunately, that is also likely to tell you you need to be an administrator.
To reset the password of the computer's administrator, read this article:
mh655 - Changing your administrator password