I am by no means a talented writer. I just have noticed some common mistakes among some young authors and I wanted to give my advice/opinions.
1. proofread! There can be so many spelling mistakes and run on sentences in stories. I am victim to this as well, I need someone to proofread stuff I hand into school. I can also understand that sometimes asking people to read over your stories can be a bit awkward. If this is the case, I would recommend reading it to yourself out loud.
Example sentence: I went to the movies yesterday it was super fun. And I went with my friends then we went to go eat food.
Correct example sentence: Yesterday, my friends and I went to the movies. It was super fun and after we went to go get food.
2. Fiction vs non-fiction. If you're trying to write a fictional story, using certain types of language can take away what you're trying to portray. For example, sometimes in fiction you will see an and after a period. This is technically wrong, but authors will still use it. If it is written in non-fiction, it will be taken as a silly mistake. Either way if you hand it into your teacher it will most likely be marked as wrong.
3. Narrative stories. These types of stories are great, especially if it's written well. Try not to describe each and every move the protagonist makes.
4. This goes without doubt but make sure to use the five W's (who, what, when, where, and why) to help people understand the point of the story.
5. punctuation. This kind of ties in with number one, but this is really important. Use different types of punctuation with different types of sentences.
Example sentence: "Woah! are you sure you can do that? It doesn't look safe." he said. "Of course I can, i'm a professional." she said.
6. The dreaded Mary-sue and Gary-stue. Personally, I think it's fine to have a Mary-sue type character. Just make sure they do have some flaws and reasons for their actions. Instead of saying that the character does something because of being super special, try to explain why and what makes them know.
7. Know your audience. This is pretty self explanatory. If you're writing a horror story, build suspense and make the audience feel on edge. If you're writing a fan fiction about a certain character, make the character act exactly like they would in whatever their from.
8. Talk to one of your teachers. If you're taking English, depending on the grade you will most likely do something involving stories. Your English teacher studied literature, they are bound to have some writing tips.
There are a lot more tips, but these are just some basics. In no way am I forcing whoever decides to read this to follow these rules exactly. Be creative and have fun. There will always be somebody out there who loves what you do even if you get hate.
Here are some links to articles you can copy and paste into the search bar for more advice.