Since there is no airport in Khao Lak you will have to take a flight to a major Thai city like Phuket or Bangkok and then proceed from there.
Tourists have the option of flying to Phuket or Krabi from any international airport and then going to Khao Lak from there. Khao Lak is 1.5 hours away from Phuket by road and 2.5 hours away from Krabi by road. You have the option of taking a bus to Khao Lak if your flight arrives during daytime hours. If late in the evening or night one has the option of taking a taxi which will set you back 1200 - 1600 THB
There are many options for renting cars and self driving from Phuket to Khao Lak. A ride will take you anywhere between 40 to 60 minutes. Take care if opting for self-drive and DO NOT drive at night.
A train is the most pocket friendly option to reach Khao Lak from Bangkok although it may be more inconvenient than other options. You may take a train to Surat Thani and then a bus to Khao Lak if you are coming from Bangkok. A journey from Bangkok to Khao Lak this way can take you anywhere between 10 - 12 hours.
Khao Lak does not have a proper bus start or stop point. Buses leaving to and from other major cities pass through Khao Lak and can be stopped at request. Tourists coming from Bangkok may have to change buses at Surat Thani depending on the bus operator so do check that before you get into the bus. Tourists coming from Phuket will have to take buses to Surat Thani, Takua Pa or Ranong.