There is effectively no public transport on the island, so your choices are to use taxis or to rent a car (http://www.1langkawi.com....), motorbike/scooter, or bicycle.
By taxi
By taxi
A taxi from the Airport to Pantai Cenang costs RM18. You can buy a coupon at the taxi desk in the airport. From the ferry terminal to Pantai Cenang the price is RM24. For those arriving at the Kuah Jetty and going into Kuah Town, the price is RM8.
By bike
By bike
You can rent bicycles in many of the hotels. Expect to pay between RM10-30 per day.