I have to fly to Argentina and can't afford 1st class, but i heard that people get bumped up to 1st,how can I

Fly standby and hope that economy is filled and only first class is available. It's risky, though, because you have to be willing to miss your flight.

You can also trade in frequent flier miles for an upgrade. If your flight is overbooked, they may ask people to give up their seats. They are willing to deal. Ask them for a first class upgrade in exchange for relinquishing your seat.

Just get lucky as far as I know. Or if you book a ticket on a day that's bound to be busy travel day, then usually you can get some kind of deal, Cause They generally overbook flights, on the presumption that a certain percentage won't show up. so if it's overbooked then someone needs to get bumped, which is usually a good deal for the bumpee.

This happens when the flight has been over booked. Happened to my dad once. I think it happens when the other classes are full but first class has empty seats. Its expensive so first class isnt always fully booked and in that case if the other classes are booked to the max they may upgrade some people.

It麓s not that you can always get a free upgrade to first or bussiness class.
As it麓s been said, this only happens when the flight is overbooked (or when there麓s a cancelled flight and the airline has to find you a seat in another flight of the same company), and there are still seats available .
however,It depends on the airline .I wouldn麓t be praying for an overbooked flight, though.
Argentina has only one main airline for domestic flights (Aerolineas Argentinas) folowed by Lan Argentina, which won麓t cover most of the domestics flights.So if you have a ticket for a domestic flights, you wouldn麓t want that one to be overbooked.Plus lately, Aerolineas Argentinas has been on strike several times (and Lan Argentina was on strike last friday).So there麓s no use in having an upgrade to first class if you won麓t be able to enjoy it anyway.
Too many passengers, too little seats.