The first, and biggest question. What brand is your sub, model number, size... Why? Different brands have different needs for air space. If you build it too small, or too big, it'll sound cheesy..
What sites? Hit the one in my Sig and goto Members Pages.. Find your make and model of car and see what others have done. That's the best place to start out before you get into the technical side of box volume, port size, and etc.