when you are buying something from somebody as them for their payment method, if they say paypal you need their paypal address. When you get this log into paypal (presuming you already have it, if not just hit the register button and follow the steps) and select send money. Enter the person who you are buying from's email address and the required amount.
DO NOT send it as a gift (avoiding the 4%) unless they are a very trusted seller because if you do not recieve your goods there is nothing you can do to get your money back
Hope this helps, if it does do not forget the rep button