You'll need a debugger, to help you find and disassemble the ASM code in the game while it runs. VBA-SDL-H is a free one out there, but it's also extremely glitchy.
You'll also need to know how to read and write basic ASM, and you will need a program to compile the ASM code that you write. Help with both can be found here.
To be perfectly honest, I'm not the best when it comes to ASM; I merely have a lot of time on my hands. But the tutorial I've linked to should answer most of the questions you may have.