Its s huge pain in the butt.
You need to sign up for an associates account.
Supply a website that is fully functioning (no dead links/furnished pages)
Make a sale within 6 months using your associate tag, after your first sale they will start the review process.
Review process is when someone goes and reviews your website and then either passes or fails you.
After that is done (can take a week after your first sale for the review to happen) you can generate some keys.
NOTE I am SOL, as I was kicked out several times a few years ago because apparently B2B sales are against the TOS so if you are a consultant dont bother.