How do I get started doing chin ups?

One of my goals is to do a chin up or pull up. I am going to get an Iron Gym or something like it, but right now I don't think I could even do the thing where you stand on a chair to start at the top and lower yourself slowly. Which is the only advice I have seen on learning to do one, aside from an assisted machine at the gym which isn't an option for me right now.

Any other ideas on exercises that will help me build up strength to be able to do one?

I am starting the hundred pushups program today as I have been pretty neglectful of my arms thus far in my journey. The only workout they really get is from very occasional yoga, pilates, and aerobic dvds that use hand weights.