Upon moving to Buenos Aires, I did not expect the biggest (and funniest) challenge would be understanding the overwhelming amount of slang used. Additionally, I did not expect EVERYONE to be using slang, but in BA 2 year olds and great-grandmas will dizzy foreigners with their dialect. Here are the basics. (Practice them with an Italian accent and you'll be prepared.)

The foundation of BA slang, Lunfardo, which was born together with the tango in the end of the 19th century. The base of Lunfardo has a lot to do with the poor street life and with the immigration that populated the city. Thus, naturally combining different languages and cultures to harmonize the locals.

The most important words to know are "che" and "boludo." You will hear them constantly.
Che, after the great Che, is used to get the attention of a friend, like we use Yo!
Boludo comes next and is similar to Dude, Idiot, Jerk, but the locals here use it in a friendly way, "Che, boludo!" to greet friends.

Other very important words and phrases:

cara de culo—Literally "ass face." Someone who is pouting or down in the dumps might have a cara de culo.

hinchapelotas : a way to say "breaking balls", for example when someone is getting on your nerves one might say, "Che, boludo no me hinchapelotas!" Meaning, stop irritating me!

mangos—Usan como "pesos." More or less would translate in the U.S. to "bucks."
cheto—Snobby. Stuck up.
tener fiaca— To feel lazy.
barbarro—"Great," "cool," okay"...
un pucho : cigarette
guarda! : look out!
laburar : to work
una mina : a chick,a girl
un pibe : a kid
un tipo : a guy
un quilombo : a mess
trucho : fake