The connection town to take a boat to Koh Tao. You can come to here by a train or a bus.
Train from BKK: dep time from BKK(arrival time at Chumporn)
Exp 19:15 (04:10), SP22:30(05:42)
Then boat to Koh Tao
Train station 06:30 -> boat from pier 07:30 -> Koh Tao 10:15.
However you had better to take tourist night bus+boat joint services offered by travel agents in Khaosan BKK. Their fares are much cheaper than ones you can do by yourself. But many thefts are reported for the tourist i\night buses to the islands.

Touts will wait you at the train station to give flyers of travel agents which arrange the boat or accommodation. The travel agents are located in walking distance and SE from the rail station.
Buying a boat ticket and transport charge to the port(50B) from a travel agent allow you to have free left luggage service, coffee and shower until your boat leaves. Get on board early to occupy a good place and take care of your luggage.
If you come here to go to Koh Tao, the best deal is a joint tickets by a travel agents.

To the port:
taxi 50B. public bus cheaper ?B

Boats
to Ko Tao:
Night boat - dep 0:00, arr 6-8:00. ow200B
Express boat - dep 7:30 arr 9:50 400B free transport to pier.
Speed boat -7:30 -9:50 400B free transfer to pier.

Bus
To BKK: 817km, 12hrs (2007 info) A2-270-181B, 6, 7, 21:00, A1-347B, 14, 20, 21, 22:00, SV-550B 21:30