The Ezeiza International Airport services most major international flights into and out of Buenos Aires. It’s about 30 to 45 minutes from the Microcenter, or the downtown section of the city. There are taxis at the airport but unless you’re familiar with the companies that are operating them, coach service such as Manuel Tienda León is a better bet. A ride in that company's vans runs about 45 pesos. They will take you to Retiro where a smaller van will take you to any downtown address for an additional 5 pesos.
Taxis are plentiful in Buenos Aires and are cheap by American standards, and most taxi companies are legitimate, though it’s best to take care if a late night means you’ll be making your way back to your destination in an area where there is light traffic. It’s best to have a hotel concierge or someone you trust in the city give you the name and number for a trustworthy company to call for these occassions. The subway (called the subte) costs less than 3 cents per ticket but if you decide to ride, make sure you’re not carrying expensive accoutrements or wearing expensive clothes, as this could make you a target for thieves.