Por or Para

in spanish, what's the rule for which one to use?

En general, "para" indica el fin o término de una acción mientras que "por" indica el medio, el agente (en oraciones en pasiva) o bien el modo.

Cada una de estas preposiciones tiene más de 10 usos diferentes, sería muy difícil enunciar un conjunto de reglas.

I usually distinguish them by thinking about PARA as purpose and POR as cause. Of course, there are many different uses of these, but it generally works for me.

Thank you Cisa.. I didn't understand anything of what pirulito said =D

I'm a beginner in Spanish

You´re welcome. But plz note that this is a general aspect which may work, but there are loads of different uses when NOT! Anyway, good luck in learning Spanish!!!!

Perhaps the following links can be a little bit helpful:

http://www.lingolex.com/swom/wom-porpara.htm

http://spanish.about.com/cs/grammar/a/porpara.htm

http://209.85.135.104/search?q=cache:QLiR3h--MBMJ:spanishwritingcenter.cla.umn.edu/oldswc/rulesporpara.pdf+use+of+por+and+para&hl=en&ct=clnk&cd=4