As mendozanow wrote above, populares are for experienced people. The best way to get to know where to stand on the populares is going with a local (one who goes regularly to football games!).
Otherwise go to platea first and study what happens on the populares...and the areas you better should avoid. The barra brava stands almost always in the middle. Watch for them as you seat confortably on a platea the first times.
I kept going to football games in Argentina for some 10 years and have never had any problem. The platea is fine, but if you ever go to a popular take only your house keys and 2 pesos for the colectivo ride back. No mobiles, no wallet...and no belt, as it may get confiscated by the police at the entrance (specially if it is a heavy one).