DevKitARM, a subdivision of DevKitPro, is a good compiler that is widely supported for general GBA development, and as such, works great for compiling ASM. You can find a good guide to installation here. It's cross-platform, so it runs on Linux and Mac as well. Or you can use the assembler in my ZIP attachment for my tutorial.
As for debugging, your best tools are VBA-SDL-H, as described in Hackmew's ASM Tutorial, and the VBA memory viewer/decompiler. For advanced decompiling, you could attempt to get your hands on IDA Pro. (I believe older versions are free, don't ask me for specifics though, I'm not sure.), and knizz has a great compilation of routines in his IDA file.