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.
“No one knows what he can do until he tries.”
– Publilius Syrus