Singapore Changi is 24km from the city center.
Taxi - there are taxi stands at the Arrivals level at each terminal. You can book your taxi when you arrive at any of the airport's 3 terminals by using the self-service kiosks, and your driver will pick you up at the waiting area. Alternatively, you can queue up at the taxi rank; the line is usually fast-moving and is managed by a supervisor - ask them if you need a taxi that accepts card payments.
Bus - buses to the city go from the basement bus bays in Terminals 1,2 and 3. Shuttles go to the city, Changi Business Park and Tanah Merah Ferry Terminal. Inquire at the Ground Transport Concierges in the Arrivals halls.
Train - trains run between the airport and Tanah Merah station, where you can transfer to the city by taking the westbound train.
Hotel shuttle - these can be booked on the airport's website.
Car rental - rental counters are in the Arrivals hall.
Limousine - to inquire about a limousine, go to the 24/7 Ground Transport Concierges at the Arrivals halls.