Generally you will find lots of help and tutorials on the web regarding WML (Wireless Markup Language: Older black and white phones mostly) and XHTML (String HTML, most colour phones).
So I won’t reinvent the wheel here.

Getting your site to be compatible with phone browsers and with normal PC web browsers is not easy. (If you visit a WML page with your PC, your browser will usually not know how to display the page at all, unless you use Opera browser of course )
So, when I was creating a WAP site that I wanted to be displayed on all platforms (PC, mobile), I can across the HAWHAW library:

HAWHAW stands for
HTML and WML hybrid adapted Webserver
and is a toolkit to create universal mobile applications.

You want to provide a mobile service?

Accessible via WAP?

… or via XHTML?
… or via HDML?
… or via i-modeTM?
… or via MML?
… or via VoiceXML

… or better all together in one single application?

… and accessible from each “big-screen” HTML browser
… from each PDA
… and even with a Lynx text browser

… and with a built-in device simulator that requires neither Java nor Javascript?