First we have this page:
http://www.cs.rit.edu/~tjh8300/CowBite/CowBiteSpec.htm
It's an invaluable source of information about the GBA hardware. You will need it, I guarantee you that.
Then we have
www.gbadev.org
The best GBA programming community site. They have both open source stuff and a great forum for information.
I also recommend you to check out
http://www.thepernproject.com/
There are some GBA programming tutorials there. Pretty good stuff.
And lastly, I recommend you to download this book
http://www.jharbour.com/gameboy/
It's free of charge and a very good read if you are a beginner. He's pretty lousy at explaining some stuff but you will eventually get a hang of it if you just try to figure out stuff yourself.
That's all I got. Enjoy mon ami!