If you use dreamhost and from their control panel, you can add subdomains effortlessly. You want to set up your subdomain as a " mirror " of your main site, meaning the subdomain is really just pointing to your existing site.
[header] How to make your website mobile friendly [title] Set up a domain mirror. [step] If your site lives at www.myawesomeblog.com, you're going to want to set up a subdomain at mobile. Myawesomeblog.com. How you accomplish this is usually pretty straightforward but differs depending on your host.