There are many possibilities to get to Koh Tao but it depends on where you will be departing from.
You have three possibilities when leaving from Bangkok. There is the train or the bus which will take you down to Chumphon or you can even fly to Koh Samui. Bangkok airways offer plenty of departures everyday. Only one thing, when you get to Koh Samui, if you plan to jump straight on a ferry to Koh Tao, make sure you’ll be landing before 12:00. After that, there is no more ferry and you’ll have to wait til the following morning.
Check out www.bangkokair.com for more information.
There are three main companies that offer different departure times. The fastest is the Lomprayah Catamaran which leaves everyday at 07:00 and 13:00. The trip takes about 1 hour 45 minutes.
Songserm Express boat has one departure a day at 07:00 and the trip takes 3 hours.
SeaTran Discovery ferry has one departure a day at 07:00 and the trip takes 3 hours.
Check out www.lomprayah.com or www.seatrandiscovery.com for more information.
From there, you can take either a day trip or a night trip. By day, Songserm Express boat leaves at 08:00 to arrive in Koh Tao at around 14:30. By night, the night boat leaves at 23:00 to arrive the next morning at 08:00.
Check out www.songserm-expressboat.com for more information.