You need to be visible to be noticed. That's the first rule of any show business discipline. Singing is no different. Sing anywhere you can. Sing in church, sing at school, sing on the street corner or the park after school, and sing at any talent show that comes your way. You have to love to sing if you want to make it to the top. It has to be your driving force.
Find a recording studio and record a professional demo. The recording studio will cost you, but it's an investment in your career. You should have no trouble finding a local studio, or one close enough for you to travel to. A three- to five-song demo that showcases your range is good enough. You'll use that demo as your calling card when you try to land an agent or an audition, so choose your songs wisely and invest your time in the final sound of the recording.
It's never too early to get an agent or manager who can help forward your career. Agents and managers get a cut of all the work they get for you, but they have the connections to get you the auditions. The money you pay them will be well worth it. You can consult an online database of industry professionals for agent contacts.
Find as many contests as you can. Many of them have huge cash prizes, but more importantly, some offer recording contracts. American Idol is a good example. Consult the American Idol website for dates you could audition. You can also consult a list of nationwide singing contests you could participate in. There are free resources online (see Resources).