FraX was very helpful back when I started learning the ways of the GBA, you might want to look for him... Anyhow.
First things first (pimping my own projects), I made a tilemap creator a while back, it's somewhat flexible, but developed specifically for creating GBA tilemaps, and comes with exporting tools for it. You can create tiles with built-in painting tools, import them from BMPs or automatically break down screenshots (or any images, actually) into tiles and maps. If you think that's something you might need, Just ask and I'll post it somewhere
wink ... You'll find links to most of what's needed at
http://www.gbadev.org ... The forum's pretty great, too.
Starter tutorials are pretty easy to come by, I started learning from Gamedev:
http://www.gamedev.net/reference/programming/features/gba1/Though my favourite is probably the PERN project:
http://www.thepernproject.com/... And if you want to do it the assembly way:
http://k2pts.home.comcast.net/gbaguy/gbaasm.htmThis here is a crappy book someone tried to get published. It's horribly bad, but at least it provides the basics for a lot of things:
http://www.jharbour.com/gameboy/default.aspxHere are the register spec:
http://www.cs.rit.edu/~tjh8300/CowBite/CowBiteSpec.htm
If you want to write sound code yourself...
http://belogic.com/gba/index.phpand
http://www.gamedev.net/reference/articles/article1823.aspVisualHAM is a pretty good IDE (avoid hamlib, though)
http://www.console-dev.de/visualham/