there are many ways to do that, i would suggest you to

1. Look on Odoo's Document first for adding payment gateways or integrate

2. Or you can read this link https://www.cybrosys.com/blog/payment-gateway-integration-odoo as well in which they have explained in depth (may be) 

I'm unsure but this link might help you out: http://www.spaceotechnologies.com/payment-gateway-integration-in-iphone-application-using-paypal-ios-sdk-authorize-net/