Surat Thani - Koh Tao
You can fly to Surat Thani from many places and flights are normally cheaper than flying to Samui; when you arrive you can book a ferry or a long night ferry including an hour-long transfer to the ferry port.
In high season it’s best to book ferry tickets in advance which you can via their websites and allow extra time at the airports.
Bangkok - Koh Tao
Travel agents in Bangkok can arrange a combined ticket for a bus to Chumphon pier and ferry to Koh Tao, this is normally a little cheaper than a train ticket. There is a choice of ferry companies that operate daily services to Koh Tao from Chumphon: The Lomprayah Catamaran or Marine Passion operate during the day or you can catch the night ferry. Check with your travel agent which one they have booked for you.
Koh Samui Via Koh Phangan - Koh Tao
You can fly to Koh Samui from many airports around Asia including Bangkok, Phuket, Kuala Lumpur or Penang, and from there take a ferry to Koh Tao via Koh Phangan. Once you have passed Immigration you can organise your ferry tickets at the airport including a transfer to the ferry port. Depending on when you arrive at Samui you may be able to take a ferry to Koh Tao directly or stay overnight and take a ferry the next day.