The forums module allows you to create a collection of forums just as you see on this site. I'm not sure what you mean "subforums", as there is no containment of one forum inside another.
1. Add an instance of Forums Module to a page
2. Click the Add Forum link to create a new forum.
3. Repeat step 2 to create as many forums as you need
If you want to group the forums you can use more than one instance of Forum Module
Hope it helps,