Getting to Koh Tao is easier than you might think, though, being a small island with no airport it does mean a boat trip. Don't worry there are plenty of them! Click here for boat schedules.
If you are comming from the north of Thailand (most people arrive in Bangkok) you need to get to Chumphon!
If you are comming from the south of Thailand (from Phuket or even Malaysia) you need to get to Suratthani!
For trains to and from Chumphon and Suratthani check out the State railway of Thailand Click here
The Thai government runs buses all over Thailand from Bangkok. To get to Chumphon, Suratthani or even Penang via Hat Yai, they leave from the Southern bus terminal. They leave every hour or so from early in the morning till about 9pm.
Many tour operators can also arrange VIP buses, often with connecting boats or flights. Though a little more expensive they are newer, cleaner and more reliable!
For flights to and from Chumphon and Suratthani check out these