A friend owes me cash, how can he pay that debt in btc?
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.