Uber cart external payment system
Basically, you need to create a custom module and let ubercart know about your gateway using hook_uc_payment_gateway(), then use hook_uc_payment_method() to define a payment method. 
Then, you continue with defining a form that is used on checkout and finish it with creating redirect to your external payment system.

You can start with this - http://nmc-codes.blogspot.sk/2012/07/how-to-create-custom-ubercart-payment.html, it should give you a good start.