I have a question. I'm currently writing a story and I'm using the simple past as my main tense. However, I'm not sure if I should use the past simple or past perfect when I describe events that happened before the main story (for exemple, one character's childhood). It seems to me I should use the past perfect, but writing many paragraphs in the past perfect doesn't seem completely right. What do you think? Thanks.
First of all, you are right: You shouldn't be writing many paragraphs in past perfect.
Past perfect is a relative tense. It can only be used in conjunction with an absolute tense: simple past. If that isn't there in the same paragraph you shouldn't use past perfect.
Past perfect used correctly:
He went around a corner and nearly ran into a guard. He immediately drew his sword. He was glad his father had given it to him. It had once been his grandfather's sword. He slit the guard's throat before she could react.
Past perfect used incorrectly:
He had gone around the corner and had nearly run into a guard. He had immediately drawn his sword. He had been glad his father had had given it to him. It had once had been his grandfather's sword. He had slit the guard's throat before she had been able to react.
Please ignore that the pacing in my example is awful.
Your chapters and paragraphs don't have to be chronological at all times, as long as you give your readers the information they need to realize that your narrative has moved back to a previous point in time.
You don't switch to the future tense if a chapter is about events that happen after the previous one, do you? In the same way you don't switch tenses when a whole chapter or long paragraph is set before the one it succeeds. Instead you move your anchor back and narrate in your main tense (simple past in your case).
For questions about style you might also want to consult the Writing community of Stack Exchange.