Step 1: Use as many payment gateways as possible, to see how they all work, to get ideas of how your own payment gateway should work. Use it as a regular end-consumer. This means doing online purchases from many different sites.
Step 2: Use a few payment gateways for your own online shops. See how it works, what issues there are, and think of how you can improve on it.
Step 3: Approach the banks, and gather more info. You will definitely need to partner with one or more, if not all the major banks, to roll out a payment gateway.
Step 4: Develop your gateway, test it, and start selling it.
You can't simply develop a payment gateway, without having any experience with them at all. You also need to have a solid understanding of how online shops and online commerce operates, which is why I think you need to have good experience running a few online stores before you even try to develop your own payment gateway. Kinda like trying to design your engine, but you've never even driven a car before. (Of course, I'm making a lot of assumptions about you. Perhaps you know these things already.)
You will need to understand how to accept various types of payments (credit card, cheque/debit card, EFT, Paypal, and other common payment methods). You will also need to partner with loyalty programmes like eBucks, uCount, Discovery Miles, etc, to get sufficient market penetration of your gateway. You will also need to focus heavily on security, SSL, Verified by Visa, OTPs, etc.
I'd say you've got a mountain ahead of you, but it is definitely possible if you have the drive, resources and funds to commit to such a venture.