Samosir island is about 4-5 hrs away from the Medan airport. When you take a taxi from Medan or a public bus you will be drop at Tiga raja harlbour. There you need to take a boat which run every hours to the Samosir Island.
From Medan, there are several big public bus who go direct to the Tigaraja harlbour, such as Sejahtera, Intra