You can fly to Koh Samui and there's a ferry service (or speed boats) to take you to Koh Phangan. There are bus services too which go via Surat Thani, but beware the roads are pretty dangerous. Especially the overnight bus services. Just last week a Bangkok to Surat Thani bus crashed after the driver fell asleep at the wheel and hit a lamp post. 10 people (including 6 tourists) died. The locals tell me the government bus service is the safest, but avoid the overnight bus at all costs.
Phil from the safety hub almost 5 years ago