NEWtoDR: Are you coming into POP rather than STI?
If so, you can spend the night at Sofy's B&B (809 586 6411, I think), have a great breakfast and Noelle will show you how to get to the bus station to get to Santiago....Either Metro or Caribe Tour will do the job. Metro has nice buses and Caribe Tours stops at two places in Santiago, depending on where you want to go...
HB