read. read. code. read.
That's basically what you are going to be doing.
There are many ways of accepting paypal payments. The major two are:
1. Forwarding to paypal and letting them handle it, then have them return to yoru site
2. Interfacing with their API.
What is their API you ask? Only PayPal knows. They wrote it, you follow it.
Thus the best place to start is with them:
https://cms.paypal.com/us/cgi-bin/?&...developer/home
Usually vendors have "example" code that you can use. but PLEASE do not just copy and paste. Learn what the code does LINE BY LINE before you use it.
Have fun,
Dan