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You're young, you want to write. Perfect.

Keep writing.

Make friends with your English teacher. Tell him/her you want to write. If you can, see if there's a creative writing class that you can take. You might have to wait until high school for a real class, but now if you have a teacher that can motivate you to write, writing will be that much easier.

Keep writing.

Listen to someone whose advice you can trust. This might not be your parents, this may not be your friends -- they may be hesitant to point out flaws in your work.

Keep writing.

Don't become discouraged when someone says they don't like your work.

And just keep writing