Where to put what
Question: Well, I'll explain it better: I'm confused about where to put some kinds of information. For example, I want to explain each character's story, but when do I do that? I keep focusing too much on the main character's story, because it's the main plot! Furthermore, I have problems in knowing when should I put flashbacks and past information and how much should happen until the point it reaches the final fight against the enemy! I know how it begins, how it goes, and how it ends. I'm just confused about the organization of things. (by the way, I'm 14 so I'm not very experienced on this, although I already wrote one or two short stories as a kid)
First, there's nothing wrong with focusing on the main character. Lots of great novels are written from a single point of view.
However, let's assume you have a reason for wanting to use multiple points of view.
Flashbacks serve a couple of purposes.
1. To explain why a character has done what they have done. For instance, let's say a character does something strange, like betray their best friend for no apparent reason. That creates curiosity in the mind of the reader. A flashback after the incident may explain what's going on by revealing an incident that motivated the misdeed.
2. To create suspense (where the reader wonders what's going to happen). For instance, most prologues are flashbacks to the initial driver of the story -- some event long ago that planted the seeds for everything that happens later.
As you may guess, where you put the flashback is
very much a judgement call on the writer's part. Do you want the reader to be anticipating something will happen? Or do you want the reader to wonder why something just happened? You may want to experiment with the order of scenes to decide which arrangement is most effective for your story.
3. Another use of flashbacks -- brief flashbacks in this case -- is to explain what is happening now. Sometimes you just need to give the reader a little information so they understand what is going in. For instance, your main character runs into someone and exchanges a stony silence. A little background information -- such as they are old enemies -- clarifies things.
4. There are times when a writer feels the backstories are strong enough to stand on their own legs and deserve equal focus with the main story. In this case, the novel resembles a collection of separate stories, taking place at different times, that may connect at some point. Usually this is a weaker approach than having one central plot.
(As an example, I would suggest the current TV series Arrow
suffers from this approach. The backstories about the main character's history on the island are not nearly as interesting as what's going on in the present and are given too much screen time.)
I would suggest, as a guideline, that if you don't actually need the backstory for the reader to fully appreciate the main plot, then you may as well leave it out. This is a corollary to the guideline that you include what you need to convey the story, and nothing else.