OK, so by the grace of madness you managed to complete your first Joomla! installation. Yes, that burning sensation on your chest is indeed a sign of some sort of manhood coupled with hair growth. As suggested right after the installation is complete, you should completley delete the "installation" directory. Don't just rename it, there are some smart thugs out there that can see right through you and call you out on trying to be slick.
By default Joomla! security requires two pieces of credentials to allow super admin access to the CMS; username and password. 99% of Joomla! admins know one part of the default credentials to your default installation (Default Super Admin username = admin). The other 1% were handed down a Joomla! site by their daddies and haven't bothered to 
rtfm. Anyway, while credential hacking is not the most popular means of cracking your way into a Joomla! site, you should still take time out to change the default Super Admin user name.
Remember there are many ways to skin a cat but there is really no need to cat a grep.
Click Sequence #1: Site -> User Manager -> Click on "Administrator" -> Change the "Username" field.
Click Sequence #2: Home -> User Manager -> [Same as above...]
ThugShot:
Note: By default Joomla! will display the "Name" of users when displaying articles, some Joomla! admins often wish to have the "Username" displayed instead, but this is not a good practice. By doing this you're once more giving away one piece of credential for site access. If anything your "Username" should not be easy to guess by taking a look at your "Name".
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Get your thug on!