Help with subdomain

[mav3n]

I know this might sound a little ignorant, which it is, but here it is. I want to use MyBB with Wordpress. I want to use MyDomain.net to lead to my actual blog, but I want to use a subdomain of forum.MyDomain.net to lead to the MyBB forum. Kind of what Wordpress did here: http://en.forums.wordpress.com/.

I know I need a subdomain and everything, but I don't know where to put the MyBB installation in my root in order to get everything to work just fine.

So:

MyDomain.net -> Blog
Forum.MyDomain.net -> Forum

Thanks FP!