I've googled quite a bit for this but haven't been able to locate any way of doing it.

You normally have your webaddress which is something like www.yourwebaddress.com but I'd like to have a m.yourwebaddress.com or mobile.yourwebaddress.com

I'm sure it has something to do with the webserver, and so I've been looking at alternative document_roots. The website I'm developing is hosted on another companies server, and so I don't have access to the main config files.