If your trip to Kuala Lumpur will last for more than a couple of days, consider buying the prepaid MyRapid card, which can be used on pretty much all public transport modes in the city.
You can get your MyRapid card at any monorail and LRT station for only 10 Malaysian Ringgits (RM), which equals to about $2.5.
You should use your MyRapid card at the ticket gates or upon boarding a bus.