The Guatemalan Currency is the Quetzal (Q).
The official rate (18.8.2015) is:
1 USD = 7,6375 Q
1Q = 0,1309 USD.
For live information check: http://www.xe.com/currencyconverter/convert/?Amount=1&From=USD&To=GTQ
A few hotels and restaurants do accept card payments, but most of San Marcos still uses cash, so it is a good idea to take money out before you arrive.
There is an ATM in San Marcos (view on map), or you can visit San Pedro where you will find two more ATMs.
Due to a recent change of policy at the banks $ are becoming harder to change, the banks have stopped changing smaller $20 bills. You can check with Restaurant Fe and Il Giardino who have traditionally changed $.
Some hotels and restaurants will let you take money out with your card but often its very dependent if they have enough cash and they will have a maximum withdrawal with a % applied.