Ao Phang Nga National Park is in the province of Phang Nga, between the provinces Phuket and Krabi. It is north of the Ko Yao islands.
To get there, get on one of the songthaews (shared taxis) from the centre of Phang Nga to Tha Dan. If you have your own transport, drive south along highway 4, and after about 5 km, turn left onto the road to Tha Dan. There you will find the park headquarters. You can hire a boat at the pier next to the park office for a three-hour tour of the park (1500 baht and a maximum of four passengers). Kayak is additional charge of 1200 Baht for two Kayaks (2 adults and 2 children).
If you're coming from Phang Nga, you can make a tour with Sayan (in the bus station) or MT Tour (in the Muang Thong hotel, 100 m from the bus station). If you arrive at the bus station there is the usual harassment. But take your time and don't believe everything the people tell you there because they want to force or lure you to their tour operator or hotel. So do not immediately buy a ticket when you get off the bus. Talk with the different tour operators and see what's the best for you. Usually there are a couple of different trips available every day. Half day tours, full day tours, and tours with a night in Ko Panyi. There are also sea canoeing trips. Tours booked from Krabi or Phuket are shorter and are more expensive.