Lexi,
Taxi is probably the easiest way but not the cheapest. You didn't supply the circumstances concerning your arrival so other suggestions for transportation involve the word 'if'.
'If' you should only have one suitcase then the public bus would be the cheapest transport. This also depends on 'if' you arrive during the day and not at night.
Don't know if you are familiar with the SD airport but the main highway is a little less then 1/2 a mile from the airport. You can take a 20 peso motoconcho out to the main road and there on the south side, under the overpass, is where the locals catch the buses going east.
All buses going by there that stop there should be going past Juan Dolio. The price should be under 50 pesos. If you should decide to use this means then just say to the man that will be at the door of the bus, (cobrador), "Juan Dolio?' and he will answer either si or no. If he say 'si' then hop on board and enjoy your ride.
Rick