A friend owes me cash, how can he pay that debt in btc? A friend owes me some money and we agreed to settle that debt in bitcoins. How can he transfer some bitcoins to my account, without me having to pay him for it?
You only have to do a simple transaction. You have to create an address (for example, this one (which is mine): 1CnCc2Dp35BZboHE8AZvoyrQXCFFaHUvTZ) and tell him to transfer Bitcoins to that address.

You dont have to pay anything for the transaction, but your friend need to pay the transaction fee.