Find or start a band (or, if you want to go for the low-grade pop, a computer with an autotuning program and fake instrument samples would work *sigh*), and practice. When you're ready, see if you can get a gig somewhere nearby. Alternately, if you're old enough to work, you could try auditioning for a job singing, although I'm not really sure where one would go to do that. Need a sax player? lol
EDIT: @HannahY123 That's also a very good point. I've seen more people mocked for their YouTube vids than getting recording deals, though, so I personally wouldn't do it. It's up to you, though. Whatever works.