Rivas post should already help you a lot.
The arduino forums itself already have some tutorials http://arduino.cc/en/Tutorial/HomePage, but you can just google for some too. Also if you open your arduino IDE you will find the examples too. Most of them are really easy and only use basic functions, but if you combine them...
Imo the main thing you have to understand is that the arduino code doesn't use port manipulation (at least not for the "end user") but abstract methodes like digitalWrite() and pin numbers.