Before you start the training program you should (a) obtain medical advice and clearance from your doctor and (b) take an initial pull-ups test.

To perform the test, simply execute as many consecutive good-form pull-ups as you can. If you can't do even one that's okay too. After you have done as many pull-ups as you can make a note how many pull-ups you were able to perform.

If you managed at least 3 pull-ups you can start the program from week 1. If you couldn't do any or only 1 or 2 pull-ups you need a little bit more training. That's why there are also weeks -2 and -1. After you prepare yourself with these extra weeks you are ready to take the challenge and continue to week 1. You may also want to check this thread.

"No one knows what he can do until he tries."
- Publilius Syrus