I think you want to know about the process of online payment. If you want to add online payment facility in your website, you need to have a payment gateway on your website through which you can accept credit cards in your website. There are two ways to add this facility on your website either you should have a merchant account or sub merchant account. Opening a merchant account is very expensive and need paper works and third party validation, need ssl, verisign or thawte certification etc. there are many other thing which I do not know.
For smaller sites there is facility of sub merchant account you can use payment gateway of third party like ccavenue or abcpaymentgateway or 2checkout etc.
Opening submerchant account is not very costly, you can setup this under Rs. 10,000/-