The best price found from United States to Guatemala is on August 25th, 2015, travelling with Spirit Airlines. See more options, compare prices and book: search flights.
Guatemala City's Aeropuerto La Aurora (GUA) is the country's major international airport. The only other airport with international flights (from Cancún, Mexico, and Belize City) is Flores (FRS). The Guatemalan national airline, Aviateca, is part of the regional Grupo TACA, along with El Salvador's Taca and Costa Rica's Lacsa.
There are direct flights from the USA with a variety of airlines. AeroMexico and Interjet fly direct to Mexico City. Grupo TACA has flights to most Central American capitals. If you are coming from elsewhere, you will almost certainly be changing planes in one of those places.
Guatemala levies a departure tax of US$30 on outbound air passengers, which is mostly (but not always) included in your ticket price. If it's not, it has to be paid in cash US dollars or quetzals at the airline check-in desk. For some time there was a separate US$3/Q25 airport security tax, but that seems to have been dropped.