The prepositions por and para have very specific uses in Spanish. Here are some of them.

PARA

1. Para is used to indicate destination or purpose.

El barco salió para Francia. – The ship left for France.
Este regalo es para María. – This gifit is for María.
Estudia para abogado. – He studies (to be) a lawyer.

2. It’s also used to express a deadline.

Quiero el informe listo para mañana. – I want the report ready by tomorrow.

3. Para also jeans in order to.

Necesito anteojos para leer. – I need glasses (in order) to read.

4. Estar para means to be about to or to be ready to.

Estoy para salir. – I’m ready to leave.
Está para llover. – It’s about to rain.

5. Para is used to express a comparison of inequality.

Para un norteamericano, habla muy bien español. – For an American, he speaks Spanish very well.

6. In Latin America, to say (minutes) to the hour:

Son diez para las cinco. – It’s ten to five.

POR

1. Por is used to indicate different types of movement in relation to a location.

Viajaron por Francia. – They traveled around France.
El barco pasó por la orilla. – The ship passed by the shore.
El ladrón entró por la ventana. – The thief entered through the window.

2. It also has the meaning of in behalf of, in favor of, instead of. Notice the difference with para in this context:

Compré el regalo para Juan. – I bought the gift for Juan. (I’m going to give it to him.)
Compré el regalo por Juan. – I bought the gift for Juan. (Juan couldn’t make it to the store, so he asked me to buy it.)

3. Por is used after verbs ir (go), mandar (send), volver (return), venir (come) etc. to show the object of the errand.

El muchacho fue por agua. – The young man went for water.
Vine por el médico. – I came for the doctor.

4. Estar por means to be inclined to, to be in the mood. When referring to a person, it means to be have a crush.

Estamos por salir. – We’re in the mood to go out.
Creo que mi hermana está por ti. – I think my sister has a crush on you.

5. Por is also used to express manner, means, or motive.

La madre tomó al niño por la mano. – The mother took the boy by the hand.
El paquete llegó por correo. – The package arrived by mail.
Los esclavos tuvieron que luchar por su libertad. – The slaves had to fight for their freedom.

6. It also means in exchange for.

Ella me dio cincuenta dólares por el trabajo. – She gave me fifty dollars for the work.
Juan cambió su coche por otro nuevo. – Juan traded his car for a new one.

7. It expresses opinion or estimation.

Pasa por nativo. – He passes as (for) a native.
Lo tomó por tonto. – He took him for a stupid person.

8. As a correlation between operations and units of measure, or expressing fractions.

Los venden por docenas. – They sell them by the dozen.
Mi coche nuevo va a más de 130 millas por hora. – My new car does over 130 miles an hour.

9. In math, it is a multiplier.

Eight times five equals forty. – Ocho por cinco (son) cuarenta.