There are several ways to approach this, varying from "quick" to "kinda convoluted" depending upon your needs and expertise, and which way is best for you depends on several things:
1) Did you use the one-click installer, and if so, do you need to be able to use the one-click installer to "upgrade" Joomla! later (or will you be upgrading it yourself "manually).
2) As lensman asked: Do you need the Joomla! generated urls to reflect "example.com" as opposed to "example.com/portal"?
3) How comfortable are you working in the *nix shell, and manipulating configuration files?
4) Are you comfortable importing/exporting MySQL database contents?