If I had £1 for everyone who says "I need to make our website work on mobile but can't start from scratch!" I'd be rich.
Here are 3 methodologies to optimise your existing site for mobile devices. They're not mutually exclusive; you can pick and mix.
There are also a few performance tips for good measure.
Lots of resources linked from http://my.opera.com/ODIN/blog/2011/10/03/over-the-air-2011-slides-web-anywhere-mobile-optimisation-with-html5-css3-j (http://bit.ly/pw6suF)