Hi,
In order to use payment services,
1) you have to decide which payment gateway are you going to use. Here is a sample comparison:
http://www.ccavenue.com/content/comparative.jsp
2) On selecting the gateway, there will be a set of API and the documentation for the integration will help you integrating your application.
Meanwhile,
you can checkout this article to understand how it will work:
http://www.west-wind.com/presentations/aspnetecommerce/aspnetecommerce.asp
Regards,
Sam.