Let me get this straight, you have a website. And you're wondering how to create subdomains on it, correct? If that's all you're doing then it should be pretty easy depending on your setup. What type of hosting are you using? A lot of hositng providers will give you the option and do all the nasty, behind the scenes stuff for you.
Just to make sure, look at https://www.hackthissite.org/forums
. Now look at https://forums.hackthissite.org
. Same thing, the second subdomain is just forwarded to the forum index.