I MADE A ROM HACK ^^;;

[Evil Robot]

A good way to get practice at using a hex editor would be to start by editing apple IIe files since thier pure ASCII and you dont really need to know anything about how hex notation and charecter sets work. Most editors can handle searching for "press return to size up the situation", and most people can change that to "press return to fuck up the situation".
But I must admit, after skimming through chojins link, there's a shit load I don't know (yet).