If you are thinking of google search engine only, just jump to the link http://www.google.com/addurl and submit your forum. The google spider will visit your site in 4-8 weeks. The other search engines like Yahoo! works the same way.
If you are looking for a faster inclusion, and if you have the money, approach a professional search engine optimization company. If your forum software supports it, you may also consider pinging other blog servers like http://technorati.com or http://weblogs.com or http://blo.gs.