Hi Filipe!
I think a good start is always the Windows Mobile Developer Center athttp://msdn.microsoft.com/en-us/windowsmobile/default.aspx
Or if you want to start the development for windows mobile 7, the windows
mobile training kit could be very helpfull:
http://channel9.msdn.com/learn/courses/WP7TrainingKit/
If you want to build something in Silverlight for windows mobile 7, this
project could be of interest:
http://videoshow.codeplex.com/Wikipage
Maybe there are more projects - I didn't do a deeper search by now. But with
the given links, I am quite sure, that you could find some more links on
your own.
With kind regards,
Konrad