As GUWonder posted, you need to have an unlocked phone. Take the phone and your passport with you when you go to buy a SIM. Activation takes overnight, but hopefully the person that sells you the SIM will be able to try out a working SIM to verify that your phone will work.

In Argentina most people I talked to called the SIM a "chip".

You can recharge MoviStar phones at businesses displaying Pagofacil or Rapipago logos (these are places where you can pay bills). Often they will give you a plastic card with a barcode that is associated with your phone number so that you just need to have them scan that when you need to recharge in the future.

The various cellphone vendors have web sites with the pricing/ recharge amounts etc - in Spanish.

John