Yes, you register as an affiliate for the companies or products that you want to promote.
Ebay is the affiliate. You get a commission on their fees that they charge the seller and you get a bonus if someone new signs up to ebay through one of your links.
No, SBI can't be paid monthly. It is a yearly fee. They do have a 100% 30 day refund and then pro-rated refund for the rest of the year.
It's not something to go into lightly. You have to be pretty sure you really want to create an online business and are committed to putting in the time and effort. That's why they don't offer the monthly payment options. It helps cut down on "tire-kickers" to some extent.