You can save money and take a direct flight from Bangkok to Suratthani, on the mainland then a bus and then a boat (the opportunity to discover Koh Samui by sea)
Thai Airways and Air Asia have daily flights. Air Asia offer the least expensive flights as well as combines: air + bus + boat (all inclusive). More information on their website.
By Bus: Both air-conditioned buses leave Bangkok’s South Terminal and Surathani several times a day. (11 hour trip). We recommend the company 999 (Government buses, very comfortable and cheaper than private). Buses ferry combination tickets are available in most travel agencies in Bangkok.
By train: Several trains leave the station Hualamphong Bangkok to Surat Thani every day. The journey takes about 12 hours. The train-bus-ferry combination tickets are available in most travel agencies in Bangkok.