Hackmew's compiler, or Karatekid's one. I can link you to Hackmew's, but I dont remember where Karate's is. Anyways, here is Hackmew's compiler, I dont know what other tools you would need, unless you bought No$gba Debugger. You can always use VBA's debugger, yeah, I think thats really all you would need.
Btw, I'm not that good at ASM, but I can explain commands to you, not tell you how to use them(you can see that I'm not very good if you see the VM's between me And FBI agent)