Taxis are easy to find as there are so many of them on the streets with their tell tale yellow and black exterior. The upper right corner of the windscreen should have a red light on if the taxi is free and make sure they are metered before getting in. The city is divided into equal squares with block numbers so telling a taxi driver a street name and block number, eg. "Buesares 2100", or the junction of two streets, eg. "Corrientes y Florida" is generally enough information.
Taxi drivers are clever and only out to make a decent living, so if you can, try engaging them in conversation. It is customary to let them keep the small change or to tip them modestly. Most locals can spot a tourist a mile off though, so be wary with your large notes and keep change on hand to pay taxis, buses etc.
Remises (prepaid taxis) are more secure than normal taxis so if you wanted to pay a little extra for comfort, get your hotel to call you a remise.