To make a forum you need :
1. a domain
2. a host
3. the forum script

You said you have the domain so u only need the host and the forum script . There are quite a number of scripts out there and they are free. ( TAN is a Vbulletin forum , you have to purchase the license either yearly or lifetime , for more information visit : www.vbulletin.com ) .

Here are some good forum scripts that are FREE :
1.phpbb
2.SMF ( i like this forum becuz their mods are easy to install )

This is just a brief answer to ur question . If u have any further questions u can post it here and i will try to help you.