Here are a few ideas which might help with your writing.  One of the things you need is raw material. Jokes, little stories, places, people and the sorts of words we use. You also need feelings. You know - sadness, joy, anger, fear, embarrassment and worry. Where do you find this raw material? In your own life, that's where.
People say to me "You must have an interesting life, Paul Jennings".  Well, yes I do, but so do you.  Everyone has something worth writing about.  If you think hard, you'll find there are countless stories in your life.  What your dog did.  The time you ran away from home.  How adults don't understand what it's like to be a kid sometimes.  How you hated speaking in front of the class.
All your experiences are fantastic raw material.  Remember, if it made you cry it will make someone else cry.  If you are embarrassed, other people will be too.  You take these feelings and exaggerate them a bit - make them larger than they really were. Once, I was embarrassed because I forgot to change out of my old painty trousers into clean ones when I went to a posh dinner.  Embarrassing - but not good enough for a story.  I changed it to a boy who is embarrassed because his head got stuck in a toilet seat and he can't get it off.  That would make people in the restaurant sit up and look.
I am nearly always the main character in my stories.  Lots of the events are straight from my own childhood and others are things that I wish could happen.  Like, being able to fly or read people's thoughts.
Before you write the first line, think.  Is it interesting?  I never start my stories with ‘It was a sunny day and the sun peeped out from behind the clouds’.  That's no good.  The reader has already closed the book or fallen asleep.  Go for an unusual start. How about ‘I did not eat your jeans. Well, not on purpose anyway’. Or ‘My cat laughs every time I sneeze’. Or maybe ‘Is this the way to the Hell's Angels Flower Show?’. You can probably do better.  Why not try?
Another very important thing is the title. It may be the best story ever, but if the title is so boring that people don't even start to read it, they will never know. So a lot of thought has to go into the title. I don't usually choose a title until my story or book is finished. 
 My first stories were terrible. I can't bear to read them any more.  If you keep practising you get better.  Read, read, read.  If you're not a reader you won't be a writer.
Don't let anyone tell you that you are just a kid - your life is as important as anyone else's.  When you tell your own stories they are real.  Add a bit of fantasy and you have MAGIC!!

Good luck!!