How do I link Paypal into my website

I have BP installed and Membership. I have also listened to the video installations several times but I still don't understand how I get to link up the Paypal express payments into my website. I have got the buttons from paypal itself but this would mean that the new members subscribed won't get to the new content which was hidden. Can someone please tell a complete beginner what to do with membership so it all links up together with the membership plugin and paypal! Thank you in advance.