Hello
1. If you are a seller you should configure and activate a payment provide you want to use to accept payments manually in Account -> Settings -> Paymants. Until that nobody can pay you. So, Credits doesn't need to be configured. You just need to activate it. And this setting allows to auto-activate Credits as payment provider for all site users just after a profile was created.

2. You may do it in Studio -> Credits -> Bundles. You may create different bundles with or without discounts. For example,
Simple: 10C for $10;
Lucrative: 100C + 20C as bonus for $100.
Site members would be able to purchase them in User End -> Top Menu -> Credits: http://your_domain.com/page/credits-home

3. Credits is like internal money. User may pay using them in Market, purchase membership level, etc. Creadits cn be purchased and withdrawn. They cannot be got for some activity on the site. It's the main difference between Credits and Points.