This is one of the cheaper options to get from Bangkok to Koh Tao. There are numerous companies that offer this service, with the Lomprayah being the most popular and reliable. Usually the bus departs from the Ko San Road area around 9pm and arrives in Chumphon at about 5am. The bus is large and has reclining seats. It will also make a few stops on the way down and service areas where you can stretch your legs and get food and refreshments. Once in Chumphon, you will be dropped directly at the pier where you will have to wait a while to catch the 7am Lomprayah High Speed Catamaran. The boat journey is nice and quick, usually taking around 1 hour 45 mins. You can sit inside, outside, or even pay 150THB extra to enter the more spacious VIP seating area. The Lomprayah arrives to the main pier in Mae Haad (Koh Tao). If possible, try to avoid booking your ticket down with the Songserm Boat company as they are much slower and have quite a bad reputation. Tickets can be booked online or via the numerous travel agencies around the Ko San Road area.
The sleeper train is also a nice cheap way to get down to Koh Tao. With this option you are more likely to be able to sleep than on the bus. Leaving from Bangkok train station at around 9pm, you will have your own bed for the night. The standard fan compartment is fine, but I would recommend booking the bottom bed, as it has much more room. The train will arrive at Chumphon train station where you will get a bus to transfer you to the pier. Yet again, try and book a joint ticket including the Lomprayah boat. The sleeper train usually gets fully booked very quickly, so advanced reservations is the best idea.
The first and cheaper of the two flying options is with Nok Air. Nok Air flights leave from Don Muang Airport in Bangkok and go directly to Chumphon. The plane is quite small and does have baggage restrictions, so check the website to check details and prices – http://www.nokair.com. Once in Chumphon you can get a transfer from the airport to the Lomprayah pier taking around 40 mins.
The more expensive option to get to Koh Tao by air would be on a Bangkok Airways flight. The flight leaves from Bangkok International Airport and arrives to Koh Samui. The plane is much larger than what Nok Air offers and you have more allowance for your baggage. Once you arrive to Koh Samui, you can get a taxi to the Seatran Pier (5 mins by taxi), then it is a 2 hour boat journey to Koh Tao. However, the last Seatran boat leaves at 1:30pm, so factor this in when booking your flight down.
The biggest bonus with taking ether of these flight options is that instead of spending 8 hours in a bus or train, the journey takes just 1 hour 30 mins by plane.