Get a C2B(collections) paybill from Safaricom if you don't have a paybill by walking into any Safaricom shop, or apply online 

Get access to the Mpesa portal. Safaricom usually gives access most of the time. In case, you don't get access to the portal write an email  to: M-PESABusiness@safaricom.co.ke and request for a business administrator to be created on your paybill, learn how to do that from here. 

Once you have access to the Mpesa portal, log in  and  create for us(Africa's Talking) an API operator/user share with us the credentials (the API operator's username and password).  Learn more about that here.