Thailand forum

Plenty of small travel agents and probably your place of accommodation will be able to sell you a combination ticket from Lanta through to Tao.
This will probably involve one of the minibuses off Lanta via the vehicle punts to either Krabi or Trang and then another minivan or big bus to Surathani or Chumpon from where you will catch a ferry to Tao.

Problem is, even with an early start you may not make the last afternoon ferry to the island - google Lomprayah, Seatran and Songserm for times. No worries, there are night ferries from both Surathani and Chumpon.

An alternative if you want to do it independently is to catch the public minivan to Trang, ask to be put off at the station (most vans terminate here) and catch the afternoon train to Chumpon. This should get in early enough for one of the night ferries to Tao but if you don't fancy a night on a mattress in a boat an alternative is to stay the night in Chumpon and catch Lomprayah's early fast ferry (I notice the Lomprayah office is now virtually adjacent the station - turn left on exit).
You can also exit the train earlier at Surathani's station but this is someway out of town (Pumpin??) and it is much further from the Surathani piers to Tao than from Chumpon.

If you don't fancy train travel, get the minibus to Krabi bus station (Krabi is closer Lanta than is Trang) from where there should be plenty of buses to both Surathani or Chumpon.