You need to head over to the learning centre and help sections to get started. (Responding to another reply you've got here - you don't need to spend any money to earn through these affiliates.)
Unless you have your own well established site or already have an ePN account, eBay is a no no right now. They have become very strict about who they will give an affiliate account to.
Amazon is still good for hubbers. The link to sign up is in the making money section of the FAQ (help page).