The easiest way to get to/from the airport is by taxi (M$200).
Buses 7 and 8 provide a good tour of the center. Bus 8 from the bus station (M$4) runs directly to the cathedral. Heading out of the center, buses go south on Villalpando. Bus 7 runs from the bus station to the corner of Avs González Ortega and Juárez. Taxis from the bus station to the center cost around M$30 to M$40.