This is the best advice I can give. When you read aloud you find out how much can be cut, how much is unnecessary. Don't even think about it now. Just write for yourself. If you want to share your work show it to family and friends.
Some schools publish young authors' work in newsletters and magazines.
Or you can ask your librarian to help you find out which magazines accept original work from children. This is a national magzine based completely on teen-generated material. Check it out. Study the marketplace. Your library should have copies of Literary Marketplace and Writers Market with names and addresses of editors and agents.