I believe on the page in your account where you can enter the various ID's, it gives information about how to apply to the various programs.
Or if you do a Google search for the program and then 'affiliate' it will probably be the first page that comes up in the results. For example:
And so on.
Yes getting approved for eBay is one of the more difficult programs. The tip I have heard is to wait to be declined, and then send them an e-mail explaining how you plan to promote them.