KTEL Santorini Buses has a good website with schedules and prices. Tickets are purchased on the bus. Fira is the island's bus hub, with departures for most villages, as well as the airport and ferry port.
Car & Motorcycle
Having your own wheels is the most convenient way to explore the island during high season, when buses are intolerably overcrowded and you’ll be lucky to get on one at all. Be very patient and cautious when driving – the narrow roads and heavy traffic, especially in and around Fira, can be a nightmare.
There are representatives of all the major international car-hire outfits, plus dozens of local operators in all tourist areas. Good local hire outfits include Damigos Rent a Car and Tony's Rent a Car. You’ll pay from around €50 per day for a car and €25/30 for a scooter/four-wheeler in high season, but it pays to shop around. Note: scooter hire requires you to have an appropriate licence, while four-wheelers require a regular car licence. Check this website for details: www.santorini.com/rentals/motorbikes.
Fira’s taxi stand is on Dekigala just around the corner from the bus station. A taxi from the port of Athinios to Fira costs €10 to €15, and a trip from Fira to Oia about €15. Expect to add €2 if the taxi is booked ahead or if you have luggage. A taxi to Kamari is about €15, to Perissa €18 and to Ancient Thira about €25 one way.
Santorini Transport is a good option for arranging fixed-price transfers to/from the airport or Athinios port.