On Apr 21, 2015 Google will update it’s search algorithm to score sites that are mobile friendly higher in search results than those that are not. The aim of course is to spur site owners to update their sites to include a mobile version. Updating a site is straightforward but in many cases doing so may require complete rebuilds. I highly suggest looking into using bootstrap to create a web responsive site that is both a desktop and a mobile version in one with no need to create a separate mobile site.
If you’d like to check if your site is ready, try using google’s mobile friendly test.