I have worked with PayPal myself - everything your looking for is in the PayPal developer guide - presumably your using a Buy Now button (or similar) so look at the documentation for that. I'll admit it's not easy, in fact it's an absolute pain trying to understand their stuff, but really (and unfortunately) there's nothing better and it is far too much/too complex to put into just one answer. This link ought to help get you started:
Website Payments Standard Integration Guide[
^]
Hope this helps,
Ed