Your question is extremely vague... if you care to specify further I may be able to be more helpful. Some basic things you need to be sensitive to: screen-size, making interactive elements large enough to operate with touchscreen comfortably, technology (most notably Flash) that may not be supported, as well as all the usual web usability guidelines. But above all, start with clean, valid HTML/CSS.
It sounds like to just dive in, so try playing around with some of the tools that are out there for mobile development.
Adobe Dreamweaver has a tool called Device Central which will show how your code will render on 100s of different mobile devices, including the Apple products like iPad and iPhone. I found this very helpful when I was creating a template layout for our .NET applications. I created the HTML and CSS in Dreamweaver and optimized it all for the target mobile devices, then integrated my code into our .NET solution.
Do you like to reverse engineer things? Try looking at various sites via iBBDemo http://labs.blackbaud.com/NetCommunity/Page.aspx?pid=249 and use the view source option in your browser to see how things that work and don't work are done. Maybe you can derive the best practices from there.