Hoo boy. Okay. I highly recommend you watch Gopher's series of videos on modding the game first. He recently did a series on Fallout 3, so I recommend watching those. Modding Bethesda games is something with rules you have to learn, but once you do it's pretty easy.
I haven't actually watched his FO3 series myself because I was pretty clued up on it already when I modded my game. Trust me, it's worth learning how everything works before you start playing, otherwise you'll end up with gamebreaking bugs, or your game just won't run at all.
If you need any help with stuff just message me on Steam or whatever.