banking systems

i was wondering if anyone here who has dealt with online payments can point me to any helpful websites (and specific pages) that i can read and learn from about the following systems:

Paypal, Protex and Secpay

At the moment my pressing priority is Paypal, but eventually i need to know them all. Our lead developer left before teaching me any of it, and i need to be making websites that interact with the above systems. So the sooner i can get reading, the better, heh.

So if there are any pages on the net that do a kind of step by step setup, they'd be *really* handy :)

Any pointers and url's to share? :)

paul.