You can buy a joint Train/Bus/ferry ticket with overnight sleeper for 1200 bt + from Bangkok.
The night train goes to Suratani then the bus to the ferry pier then ferry to Samui where you get the ferry to Phangan.
You can get a Bus combo ticket fron Bangkok to Chumpon and the ferry will take you to Koh Tao then Koh Phangan and Samui last about $40.00
If you are going to the FMP in Jan you will need to book accomodation well in advance as this is the high season and a lot of places are full on Phangan and Samui.
Have a look at
Sawadee.com - Thailand tourism portal - Hotel and Resort reservations to get an idea.
Den