Do your usual install on wordpress, and get it up and running. If you are not sure what that is, download the zip, upload the contents to your website, then point your browser at it to finish the configuration.
Once you have filled out your database information, and sorted themes etc you can then move onto the next step. Do not enable any plugins at this stage, some may be incompatible with multiuser!
Look for the file wp-config.php on your server, you need to edit it and add in one line to it.
Once you have added it, refresh your dashboard and go to Network settings under Tools section. This will give you two Options
* Subdirectories as in http://yourdomain.com/someuser/
* Subdomains as in http://user.yourdomain.com
Most people will want to use subdomains, This will require a wildcard DNS entry however. That means just add an A record of * pointing to the IP of your server, then add in the apache config below before restarting.
Once you have done this you can go under the SuperUser menu and add a site in.
Some extra wordpress notes:
* make the plugins/themes folder writable by the web server. It needs to be owned by the same user not just chmod 777/666 for some reason on a lot/most systems. This is so it can read/write plugins etc without the FTP details. Using the FTP is a more secure option, however it can be painful and annoying to do.
* Not all plugins are compatible with multiuser
* Security can be a problem, one site gets compromised, all sites may be.