I'd say start fiddling around with stuff in gameboy games. If you're really ambitious, start reading up on ASM and try to apply it in gameboy games. ASM really something you need if you want to go really far with romhacking.
don't try to do everything at once though, read a LOT, and play around on simple-ish systems like NES, Gameboy, Master System, etc. something you are familiar with would help too.