I sometimes see modified NES games, and I'd like to try my hand. What tools are used to accomplish this? Is there some magic repository of source code sitting around somewhere that I don't know about? My question is, what is step one to modifying any arbitrary NES game? (If step one is something like 'get your head on straight', then please give me steps one through three.)
Thank you
What you're talking about is called "ROM hacking."
Now you'll want to be careful here because there are serious legal issues (note, I am not a lawyer): For one, to hack a game, you need to find a ROM image of the game you're trying to hack. There are sites that provide downloads for these, but I will not link to one. You're on your own here. The other issue is that a hacked game would count as an infringing derivative work. In practice, however, the game companies mostly ignore the ROM hacking scene.
Legal issues aside, these are the essentials for ROM hacking:
Romhacking.net hosts other useful tools as well.