'Por' and 'para' convey very different meanings in Spanish and using one instead of the other can change the whole meaning of the phrase. For example:-
Trabaja para Carmen - She works for Carmen (i.e. Carmen is her boss)
Trabaja por Carmen - She's working in place of Carmen (i.e. she is performing Carmen's duties because Carmen can't make it into work today)
Uses for 'por':
-Expressing the cause (not the purpose) of an action: Me caí por la nieve. I fell down because of the snow.
-Expressing movement along, through, around, by or about: Anduvimos por las calles de Jaén. We walked through the streets of Jaén.
-Expressing time or duration when something occurs. Viajamos por tres semanas. We're traveling for three weeks
-Meaning 'for' in the sense of supporting or in favor of: Trabajamos por derechos humanos. We work for human rights.
-Introducing the agent of an action after a passive verb: La novela fue escrito por Almudena Grandes. The novel was written by Almudena Grandes.
-Indicating means of transportation: Viajaré por avión. I will travel by plane.
Uses for 'para':
-Meaning for the purpose of or in order to: Hay que estudiar mucho para aprobar los exámenes. You have to study a lot in oder to pass the exams.
-With a noun or pronoun as object meaning for the benefit of or directed to: El regalo es para usted. The gift is for you.
-Meaning to or in the direction of when referring to a specific place: Voy para Europa. I'm heading to Europe.
-Meaning by or for when referring to a specific time: Necesito el coche para mañana. I need the car for tomorrow. Vamos a la casa de mi madre para el fin de semana. We're going to my mother's for the weekend.