Rome2Rio will give you different modes of transport you can use to get to Koh Samui from Bangkok. Your choice will depend on your budget and time. We used the Train-Bus-Ferry option. We bought a joint ticket at the ticket counters in Bangkok train station.
We took the overnight train from Bangkok train station and arrived in Surat Thani in the morning. We got off and were given stickers written “Koh Samui”. We then waited like an hour for a bus. Within that hour we had agents trying to sell us everything from hotel rooms to transfer taxis to breakfast to souvenirs!
There were many buses but each one had the name of its destination written infront. We travelled 1 hour to get to Don Sak pier. The bus was airconditioned and comfortable.
For the last leg of the journey, we used a ferry. It took 1 h 15 minutes to get to Koh Samui.
From Koh Samui to our hotel in Chaweng
At the pier, there were mini vans charging 200 Baht pp and “meter” taxis charging 700 Baht pp. We were not in a hurry so we walked out of the pier and towards the road. There were local taxis (Songthaews), charging 100 Baht pp. Those were the ones we settled for and they dropped us right at our hotel.
In order to get the Songthaew to stop, you have to press the button on the roof of the vehicle or bang on the roof to get the drivers attention! A Songthaew fare can range from about 20 to 60 Baht and this is paid to the driver when you have just exited the vehicle. However, tourists are usually charged 100 Baht per trip.