WordPress 3.0 has multiuser capability. What this means is that you can set up a network of sites with a single WordPress installation.
While 3.0 is multiuser capable a couple of files need to be edited and some settings checked in order for the network to actually be available.
You should know how to use ftp and use a text editor to follow these instructions. I also recommend backing up the files that you are going to edit before making any changes to them.