The currency is Sierra Leonean Leone (SLL), however in Freetown you can pay in dollars. Be aware that shops, hotels and restaurant only accept notes dated from 2009 and onward.
Make sure to always have cash on you as many places do not accept credit card, or quite often the credit card machine does not work.
You can however safely take out money from the ATM machine Ecobank, you can find them all over the city. Be advised that sometimes the ATM might run out of cash.