By default the Joomla administrator account is called "admin". To protect youself, and make it harder for anyone trying to guess your account details is also recomended to change the admin's username to something more difficult.
To accomplish this:
1. Log into the Back End
2. Select User Manager
3. Select the 'admin' user record
4. Change the value in username. (Good user names contain a mix of letters and numbers.)
5. Save
6. Remember the new user name!
If you ever forget the admin user password (or someone breaks in and changes it on you), see this article. INSERT ARTICLE ADDRESS