Well, why not start and the beginning?
http://doc.trolltech.com/qtopia4.2/index.html
There you will find:
How to install:
http://doc.trolltech.com/qtopia4.2/i...ng-an-sdk.html
how to build:
http://doc.trolltech.com/qtopia4.2/howtobuild.html
And many other answers.

Good luck!