This might not be related to the game you specifically want to mod, but take it from a complete LUDDITE who didn't know what hex editing was until a few days ago - I found this INVALUABLE.
http://www.youtube.com/watch?v=aP3pZU6Ahlo
It should teach you a couple of basic ideas. I still don't have a scooby about what I'm doing but I've managed to get the results I wanted pretty quickly for an old fart with no prior knowledge