When you install Joomla!, the default username of your login account (the critical account of the almighty Superadministrator) is admin. By changing it to something different, you'll make it much more difficult for bad guys to gain access to the site. After all, they won't be able to guess your username anymore.
You can't change the administrator username during the Joomla! installation. After the installation is finished you can enter a different username. This is how you do it:
1. In the backend of the site, navigate to Site | User Manager.
2. Select the Administrator user record (Username: admin).
3. In the User: [Edit] screen, enter a new Username. Be creative!
4. Click on Save to apply changes.
Log out and log in to the backend with the new username.