If you are after a site that responds to most devices such as iphone, android, ipad, tablet, nexus, then you can download and install these themes and see how easy it is to reformat your site to make it more user friendly on a mobile.
Website to mobile website (this one was really easy and looks great, see it in action @ bestbuys4business.com)
You can also use plugins suitable for WordPress, these are the ones I have used and recommend. There are heaps to try, give them a go, one by and test them to see which suites your theme best.
Some experts are advising we do not need to have a separate mobile site. That is way it started out but that is shifting now with WordPress making it so easy to mobilise your site, you don’t need a subdomain or another site .mobi to make it work.
Transform your website to a mobile website now and see how easy it is.