How can I tell if I should use the perfect of imperfect tense when talking about thing in the past?

In general, the imperfect tense is used for descriptions and ongoing\repeated actions in the past, for example:

Description - It was raining: Il pleuvait

Ongoing - I was reading my book: Je lisais mon livre

Repeated action - When I was young I used to go to the park with my friend: Quand j'étais petit(e), j'allais au parc avec mon ami

Whereas the perfect tense is for one-off, individual actions, for example:

Yesterday I went to the cinema: Hier je suis allé(e) au cinéma.

Here is an example that shows the two different tenses used together to talk about past events:

I was watching television when my sister arrived: Je regardais la télé quand ma soeur est arrivée.

Watching television is an ongoing action (imperfect tense), and the sister arriving is a one-off event (perfect tense).