Bus, train, air plain or minivan. There are many ways to go to Koh Lipe. The easiest would be to fly to Hat Yai and from there catch a minivan to Pak Bara and from there you just buy the ferry ticket to Koh Lipe... Or if you go slow, catch a sleeping train from Bangkok and save the money of one nights accommodation...
Walking path map to Bila Beach. If you want a long-tail boat from the ferry drop-of