Few ways to implement a responsive theme.
1. Modify the template pages to use a framework like Skeleton CSS (http://www.getskeleton.com/). You\’ll need to update each of the main template files with the classes that Skeleton uses. Use an @import on your CSS to include the skeleton style sheets (or add them to the head of your Web site).
2. Buy a responsive Magento theme
3. Set some mobile exceptions, create a new home page for each exception, modify your underlying page code to work on a moblie screen. Use a tutorial like this one. http://magebase.com/magento-tutorials/how-to-mobilize-your-magento-site-for-iphone-android-mobile-commerce/