First of all, you both need to have Paypal accounts. They aren't the easiest to sign up for, i haven't done if for 3 years but when i did they charge your debit cards 2 amounts under a pound which you have to enter to complete the registration (they credit you back the money once this has been done).
Once this has been done, all the buyer has to do is enter your paypal address (an email address you registered with) and send the money. Paypal do charge you for the transaction, and anything under £50 you want to move into your bank account, they charge you 99p/£1.50 i can't remember. So they do charge you the earth for using the service, but its convenience is Paypals big advantage.