Las Americas Airport to Santo Domingo

Where can we catch a public bus (gua-gua)?

you can take a taxi which would cost you about 20 dollars american or take a taxi to las americas (the hiway at the entrance of the airport)turn right and the bus depot is right there it will cost you 12 pesos to get to downtown Santo Domingo. But all this considered you would be a lot better off just staying in the taxi(shifting luggage and what have you)

The taxi to Santo Domingo will cost you RD$300. To reduce costs, we suggest you find another passenger(s) who want to share the ride into Santo Domingo. The driver will only make one stop in Santo Domingo. Inter-city distances can be reached with a RD$50 fare on taxis that you can call by the phone (lots of phone companies listed in the Yellow Pages).

doug and DR1 thank you for the info. we will be 2 adults and 1 child

Actually, Dolores, you mean INTRA-city (within the city), rather than INTER-city (between cities), is around DR$50. This is a one-way, single destination charge. [If you're picking up someone and continuing to another destination, they'll usually ask for more. Pay more, but negotiate. No matter what they say, they're willing to negotiate.] Speaking from my own four years living in SD, some taxi drivers will try to quote you more than DR$50 if they think you are a tourist, but be firm about the DR$50 and they'll usually acquiesce.
Regards,
Keith