Hello All!

I'm going to attempt to hack FFV Advance to Add a few more classes, a new boss fight, etc...
The problem is that I'm a bit of a newb at ROM hacking and don't quite know where to start.  I've had some experience with hex editors and assembly, and have altered the text in some games here and there, but have never tried anything of this magnitude, and thus am not quite sure of where to start.  Here's what I'd like to do:

In the third world, I'm going to place an area (somewhere) where the party fights Ashura.  If you beat her, you not only get her as a summon, but also you gain access to three new classes: Dark Knight, Gambler, and Artist.  Obviously, these will have abilities taken from characters in FF IV and VI of the same classes.

I'm fairly sure this will require editing the ASM code, but I'm not even sure what sort of assembler/disassembler to use for a GBA game.  I understand that this will probably be a huge project that will require a lot of practice and work on my part.  I'd just like to know where to start and what tools I will need, and figured this might be a good place to gt some info.

Thanks in advance!