[tomyws]

Familiarity with hardware would be a good starting point - the author explained how he can made use the three tables used by the PPU to pick out objects and textures. There is a lot of information about the NES online which would be useful references.

Then a good set of debugging tools, which is possible for NES games as there are many different emulators written in all sorts of languages which would make for a good learning resource.

To get more involved, investate open-source emulation projects - building them yourself. Then get more involved - try to fix an open issue - which will help you get familiar with the project. Read up on reverse engineering and ask questions!