There's plenty of taxi's waiting outside the arrival hall of the airport and you can hop into one after buying a pre-paid coupon. Should you choose to call a taxi instead the numbers are :

Comfort Cabs +60-3-62531313
Sunlight Taxi +60-3-90575757
Public Cab +603 62592020
Uptown Ace +603 92832333

The Skybus is the best and cheapest way to get into the city. You can view the schedule at www.skybus.com.my

For a more comfortable ride into the city, take the shuttle bus to Salak Tinggi and hop aboard the KLIA Transit to KL Sentral. There are packages for the bus and train ride so do ask at the counter.