There are hundreds of different companies that offer day-trips to Phi-Phi Island. They will pick you up early (7am) at your hotel with a minibus and bring you to Phuket town's harbor to board their speedboat.
There are travel agencies all over Phuket, on the airport, and you will find travel desks in most decent hotels. They will help you to book your tour to Phi Phi. A two-way ticket cost about 500 bath and that usually includes a pick-up at your hotel in southern Phuket. Transportation to the airport costs about 500 bath one-way.