bbPress.org

Put it in your root, but in a folder of its own. So, if you have WordPress here

/home/www/example.com/wordpress/

you can put bbPress here:

/home/www/example.com/wordpress/bbpress/

Then, your website/blog is http://www.example.com and your forum is http://www.example.com/bbpress/

You can call your folder whatever you want, it might be forum, forums, bbpress, anything at all.