I want to implement my own payment gateway using C#. Can any one guide me ?

Implement one or create one? If you are coming onto a forum asking how to *create* a payment platform then, with respect, the process is probably beyond you. You're going to need millions of pounds of infrastructure, you're going to need legal agreements with a lot of financial organisations, you're going to need a vast team of legal experts so that you can comply with legislation, you're going to need to design and develop a complex API.

Easiest way in to a payment platform will be to talk to a bank that offers merchant services and you can use those direct without going through third party providers. Like I said though....this is going to cost massive amounts of money in hardware, licenses, legal fees etc etc etc.

If you simply want to implement someone else's platform (PayPal, WorldPay etc) then that's much easier, simply consult the documentation from the chosen vendor. You normally need to sign up before they give you any technical details.