Colombo has Sri Lanka's only international airport Kattunayake. Almost all international tourists to Sri Lanka come via Colombo. From the airport, regular buses are available to Colombo city (every 15 to 30 minutes, not in night). The buses would carry you to one of the main stations of Colombo. For picking a cab, try the 'fixed-price' ticket system taxi counter at the airport. That way you would not have to do bargaining.