Back when I first decided I wanted to be a writer all I thought about were writing books. Since then I've learned a lot about the writing world. I still think about writing books but I have also discovered that there are a lot of opportunities for new writers. One of them being magazines.
There are many magazines looking for freelancers to do articles for them. It doesn't matter if you're 12 or 20, it is very likely a magazine out there is interested in what you have to say.
You might be thinking "I don't know what to write about." Well, I thought the same thing a couple years ago but I learned something since then that has given me confidence: write about what you know.
When someone approached me about writing for her homeschooling magazine my first thought was: "But my Mom's the one who homeschooled!" Then my friend reminded me "You might not have been the one doing the homeschooling but you are on the other end, the one who was homeschooled." And since then writing for the magazine from a graduated homeschooler's perspective has given me all kinds of ideas for articles!
If you're alive than you've had life experiences, write about those. If you've had trouble with something chances are that someone else has too and would like to know how you dealt with it.
It's important to do research on the magazine before you query them with your article/short story/poem so you know what age group and what topics they publish, if they like what you show them they might even ask you for more!