You can take the new hightway to Samana, you just have to take a bus from la Romana to Santo Domingo and tell the Cobrador, the guy that collect the money that you are going to Samana so they will drop you just in fronto of the Bus Stop, you have 2 options, Small Bus or Big Bus from Caribe Tours they have the bus stop one next to other.
Its locate in Las Americas so after the toll remind them you are going to Samana because they can take you to Santo Domingo if they forget about you.
In Samana they you need to find out how to get to Las Galeras.