When should you use the "passé composé" or imperfect tense with French verbs?

The "passé composée" is used for a single action which happened in the past and is now completed as a one off instance. Usually 

eg. The phone rang. Le téléphone a sonné.

This sentence uses the "passé composée" because the action of the phone ringing happened once and is talked about as a singular action in the text.

The imperfect tense can be used when talking about an action in the past when the verb is used to describe the setting of the passage.

eg. It was raining when the phone rang. Il pleuvait quand le téléphone a sonné.

In this example, the action of the phone ringing is the same as before (passé composée). However, the verb "Il pleuvait" acts to describe the scene on which the passage is focused.