Do a query on your favorite search engine for [mobile search optimization].
Basically there are two ways to be mobile-friendly:
1) If your site is valid XHTML, you can have instructions in your CSS that are specific to mobile devices using @mobile and/or @handheld. This sends the entire page to the mobile device and then instructs the device on how to rework the page in a mobile friendly manner (proper page layout is critical - there are best practices I won't go into here). The advantage of this approach is you only maintain once document. The disadvantage is that the mobile device may have to download more than they need (which is a real issue given their limited bandwidth).
2) Or you can insert a <link rel="alternate" media="handheld"...> in the header of your regular page to the mobile-optimized page. In this case you're saying "the content on this page is the same as this other page - mobile devices should see the other page". Again, the mobile page needs to conform to certain standards - XHTML and/or WAP/WML. The advantages/disadvantages are the opposite of the first option.
Once you have mobile-friendly pages it's a good idea to come up with a mobile sitemap and submit it to Google, Yahoo! and other services that are mobile-focused.