When should I use the imperfect and when should I use the preterite tense?

The preterite tense is used to describe single, completed actions in the past. For example, "last Monday I went to the cinema" - "el lunes pasado fui al cine". The only time we use the preterite tense for a repeated action is when it is specified how many times it is repeated. E.g. "te llamó tres veces" - he called you 3 times.
Alternatively, the imperfect tense is used for actions that are repeated habitually. E.g. "iba al colegio cada mañana" - I used to go to school every morning. We also use the imperfect when referring to somebody's age in the past - "tenia tres años". Finally, the imperfect tense is used to describe characteristics, be it physical or emotional. "Era guapa" - she was beautiful. Or, "estaba triste" - she was sad.
My key rule is: visualise a timeline - if you can draw a line on the timeline for when the action started and finished, then it is most likely requiring the preterite tense.