How do I create a multi site network?

by

http://codex.wordpress.org/Create_A_Network

Have you looked through the basics?

You can set up subdomains, site.domain.com
or subdirectories domain.com/site

Or you can even route domains to the sub sites.
Check out that stuff first, as multisite is built in to WP since 3.0

If you had more specific questions you maybe rephrase your question?