Mobile devices come in many sizes and forms. Smart devices span the range from smart phones with 3" screens to tablets with 7" screens and beyond. Firstly check whether your website displays correctly across all devices. A few tools you can use for this are:
DesignModo's Responsive Test
Responsive Test by ZooExtension
Responsive Design Test by StudioPress
In the past, sites have used separate URLs for the mobile version of the site or served different content on mobile vs. desktop access. Given the wide range of screen sizes, using a responsive layout for your site is probably the best option today.