A wild shroomish appeared!
@KanouKono - There's no such thing is a specific ASM tool, as ASM itself is unspecific, all you need is a emulator with builtin debugger(VBA-SDL-H), disassembler(VBA has one), assembler(GoldRoad, Devkit Adv, etc) and some knowledge on ASM. Then you fiddle around with it, find stuff amd modifiy it to your desire. There aren't much tutorials on ASM, google it or something, PHO has some.
There are a lot of very in-depth tutorials on ASM. It's just up to you to gain a decent enough understanding to apply it to roms. The Dark Alliance is a good place to learn basic ASM, and more complicated...Most people who know ASM well, apply their knowkledge in diffrent areas so yeh. Also Cheat engine and it's tutorial are a good place to start. Cheat engine is a memory scanner, that uses ASM. (Some AV's scan it as a trojan -_-. The site warns you).
It's just up to you to learn the basics, then apply em in roms. Btw ASM is not reccomended for a first language :)