Paypal has a vast amount of info on how to do everything they can do for you. On the client-side, there is nothing more than a button which is encoded with your special keys to make it work. The server side handles all the rest. The button uses the Paypal library to handle everything.
Instead of repeating a lot of Paypal info, here is a tutorial how to set up a simple form and post to the account. It should help you get started and then come back with your issues once you get some code to show us. Paypal-Integration Hope it helps!
Paypal also has a tutorial, or really demo that walks you thru how it works. It shows how the client side would work thru your website’s cart system and then a pop-over that lets the user pay by Paypal. You can download sample code there somewhere, too. Might help… Paypal-demo-checkout