There are a few ways to go about it, one being a whole seperate website and just forward mobile users to this website, the other is to use css to strip down the site/images/adjust widths etc, and a third option is to use javascript show/hide to hide mobile sections of your site from large screens and vice versa.
Anyway you decide to work it, the best tool out there to detect mobile browsers is
Tera-WURFL. You may decide you want PCs to use one site, mobiles to use another, and iPAD users to use yet another, this tool will determine that for you and even give you users screen res.