This is not as simple as one may think.
First, determine if the web app is accessed from a mobile device is not easily done. What is a mobile device? Is iPad a mobile device? Should all devices with a resolution less than 1024*768 be handled as "mobile devices"?
Second, designing a web application to make it compatible for all different devices - which in turn can have many different browsers installed - is not a piece of cake.
But fortunately there are some solutions as well.
Any of them can be used to switch a mobile device to a special mobile adapted site. They can also be used to build/adapt a mobile friendly site, depending on what you need.
When you have built an mobile friendly site you should first of all test it devices with different resolutions/os/browsers. Another useful site is http://mobiready.com where you can validate the site.
A final tip is to consider which devices to support. Only iPhone/Android? Maybe have a full version for these devices and a simpler version for all other devices. Depends what your requirements are.
Good luck!