When should I use the preterite and the imperfect form of the past tense?

In general the preterite is used when something has been completed. If an action has a definite start and end then you should be using the preterite, this can be both if it is something that happened once or if it is something that was repeated a specific number of times. 

For example - Ayer, fue al cine. (Yesterday, he went to the cinema) 

Me llamó dos veces. (He called me twice)

The imperfect is used when you don't have a definite end point. This could be something that happened generally in the past or something that has not yet been completed. It is also used often as a way of setting up the use of another past tense. 

For example - Cuando era niño, tuve un fútbol. (When I was a child, I had a football)

Yo estaba hablando cuando tosía. (I was talking when he coughed)