Take vocal lessons. This applies to anyone who wants to sing, but especially to teen singers. A good voice is often still undeveloped at an early age, and a vocal coach can help a teen singer develop proper singing habits early on that she can strengthen and preserve her voice over time. A singing coach can also teach proper breathing techniques and assign exercises for increasing vocal range.
Audition for singing roles in school productions. Sing in the school choir. Singing live as often as you can helps you perfect not only control over your vocals, but it gives you a chance to work on the way you present your singing to an audience.
Get a manager (see Resources). You're probably still in school if you're in your teens, so you'll need someone to help you find work as a singer. A manager can find you work in commercials and maybe even on a TV show. They do this for a percentage of each job they get for you. You'll need to record a demo to present to talent and management agencies so they can hear you sing, and they may even request a live audition.
Participate in local and national talent contests (see Resources). You'll need to make arrangements with your school to make up work, but participating in talent contests is a good way to attract attention to your singing talents, especially if you participate on a national level and win.
Make a portfolio that contains background information about you, a few glossy photographs and a list of contests you've won or singing work you've done. A portfolio, along with a recorded demo of your singing, is what you will need any time you audition for a job or contact a talent agency.