Finding a SIM card in Argentina is a pain but as long as you speak Spanish it can be done. I recommend Personal, the largest GSM provider. Stop in at one of their shops and ask for a SIM. It should be pretty cheap, about 15-20 pesos. You then fill out some information, wait a day for it to be activated, and there you go. You then buy the Personal Light cards at any kiosco to refill it with time.

One thing to beware of, however, is that prepaid cell calls are quite expensive. Peak time calls typically run about 1 peso per minute, so roughly 33 cents/min. Not a big deal for the occasional call but that can add up if you're on lengthy calls to friends all the time.