Will you be a senior this year or starting college in the fall? If you’re still in high school, see if your local paper has a mentor program. Ours has a youth section that runs once a week, and several interns are hired from the local high schools to write articles for that section. They work with a newspaper editor and learn a lot about the trade and how to actually write something of a high enough quality to be published. Several have gone off to college and returned to be hired as employees of the paper. If your paper doesn’t have something like that, approach the entertainment/arts editor to suggest such a partnership. You may create an internship for yourself. If you do, check with your school and see if there is a way to get credit for it; many schools offer high school credit for mentor programs.
You can also find a summer job that entails writing – working as an intern for an advertising or marketing agency or for a local magazine. Most places have small magazines which run on horridly tight budgets, and one of them may be willing to have you work a few hours a week at odd jobs. It’s a great way to learn the insides of the trade and to make contacts that will come in handy at some point in the future. Editors move around, so you never know when someone you worked for one summer will be the editor now at the helm of a publication that is just where you’d like to work.
Also, you’re not too young to start sending out query letters for your fiction and for nonfiction. Learn how to write a compelling query letter. Check out books from your library and practice, practice, practice.
If you are going into college, my best advice is to take what you learn seriously and get involved on campus with some type of activity that helps you explore your writing ability.