1

To get to Koh Tao from Samui you can take one of three ferries: the Lomprayah, Seatran, or Songserm. Songserm is cheapest, slowest, and least comfortable. It will take 3-4 hours I believe. Lomprayah and Seatran are both fast and quality boats. To get back to Bangkok from Koh Tao go to any travel agent on the island and book a combo boat/bus ticket, or boat/train ticket. You can take a boat back to Samui and fly but that's much more expensive. I usually take the Lomprayah ferry to Chumphon in the afternoon, then a night train up to Bangkok. If you get a sleeper car in the train it's really quite comfortable, and quite cheap. Any travel agent can help you sort out your options and make the booking for you though.

2

I second the above comment. The only difference being that we did a boat/bus combo, no train. Although I'm sure the train is much more comfortable for an overnight journey.

Enjoy.
Goats On The Road

3

For us we did Ferry+Bus from Koh Tao by Lomprayah http://www.lomprayah.com/ back to Bangkok (stop at Khao san area)