Cain and Abel is about as easy as it gets 
If you don't understand how to use it in a broader, theoretical sense, you probably need to learn more about networking in general and computer security in particular. I can't really tell you where to go to get this grounding, I just did the easy option and did a VET course on networking (CCNA), but help is only a click away.
If you are stuck on the interface, then oxid.it has a very good manual on it. I think its bundled with the program itself, but go here.
And that's the end of my link-filled idiocy!
