Requirements for making a Web Application/ Website Mobile compatible

Hi Guys

I have a Web based application being accessed as a simple website globally.
I wish to understand, are there any fundamental changes I need to make to make my website better suited for mobile devices?
Also, is there a way I can detect, if a request for the site is coming from a mobile device or a standard computer to be able to provide different responses in both cases (my understanding is some websites do so Ex- When accessing rediff.com from my computer takes me to the standard website, however, when I try accessing www.rediff.com from my mobile device, I am redirected to m.rediff.com , which is a little different in appearance from the standard website- wish to know how can we discern if the request to the server is coming from a mobile device or from a full fledged computer)

Please do forgive me if my questions sound too elementary, I am just trying to learn from scratch

Cheers
Bhootbravo

Not a JavaME question. Moving to 'General Computing'

Every browser identifies itself in the request header. You need to look up whether the browser is mobile or not and then act accordingly. Here is one library that will do this for you: http://blog.mobileesp.com/