Start a YouTube channel. Some people have actually managed to become famous through posting videos of themselves singing on YouTube.
Remember: the Internet isn’t always a kind place. If you’re not sure your voice is great, it might be wise to hold off until you have some more training. You might get accolades online, but you also might encounter people who are out to tear you down.
Additionally, it’s wise to keep in mind that the things you post online will, in a way, live forever. Only post work you’d be proud to have anyone see, and that you think you’ll still be proud of in 10 years.
Do not post on YouTube if you’re underage. If you’re a minor, ask one of your parents or manager to help you post content.