Hi there,
First of all I would suggest to you to take a look at the payment plugin which already exist in Hikashop, Paypal is a good example. You will also need the documentation of the company for which you want to code a plugin as well as a test account.
Once you have these information, you will have to make your plugin communicate with the API, once again, looking at the already coded payment method can help you.
I also suggest you to take a look at our developers' documentation:
www.hikashop.com/support/documentation/6...r-documentation.html
I hope I answered your question