Don Sak is a small town in Thailand. The only reason to come here is probably to take the ferry from or two the Eastern Islands, Ko Samui, Ko Phangan and Ko Tao.
From the ferry port in any direction
Just walk away from the port and pass the toll gates. You can stand right where the road begins and it will be easy to fetch a lift straight to Suratthani. If you are heading in direction Krabi or Chumphon, be aware that the motorway splits before Suratthani, to make sure you get off your car in time.