Um, well ASM is very useful for custom things, but to be completely honest you should probably learn mapping, scripting and get graphics before you work on the battle engine.
Get some good editors. I recommend XSE (for now), AdvanceMap 1.92 (for now), G3T for Moves, Pokemon Stats, Items, etc. Itari's trainer editor is probably the best for now.
Maybe practice inserting different routines from my ability resource thread first, and looking at how they work if you want to make new abilities, same for the new move effect and animations, look at the move resource thread. If you want Megas and the PSS split, go to those threads and apply the programs etc, etc.
Good luck.
Get some good editors. I recommend XSE (for now), AdvanceMap 1.92 (for now), G3T for Moves, Pokemon Stats, Items, etc. Itari's trainer editor is probably the best for now.
Maybe practice inserting different routines from my ability resource thread first, and looking at how they work if you want to make new abilities, same for the new move effect and animations, look at the move resource thread. If you want Megas and the PSS split, go to those threads and apply the programs etc, etc.
Good luck.