If you are a teenager, never think you are too young to start investing. All you really need to invest is money. You really don't even need to have much of that either. You can start with $ 100 if you want, you just need to minimize your expense of investing which isn't hard with discount online brokers.
First, if you are 18 or older, things shouldn't be difficult. You should be able to sign up for you own account with a brokerage firm. Some states might require you to be 21 or older, but most are 18. If you are a minor under 18 years old, you will need a little help. Ask your parents if you can set up a custodial account through them. You will then have full access and management of your account once you are no longer a minor.
After that, you should take the same steps as everyone else. Learn as much as you can about investing to start and then keep learning throughout your time investing. You will never stop learning. You will learn from mistakes and you will learn from success. The more you know, the more confidence you will have. The more confident you are, the more risk you will take and the more money you will make.
Also, make sure you keep saving money to invest. You can't invest without money, and even though you can start small, eventually you will want to go big. Starting so young will give you a huge advantage. You will learn to save and invest more, you will have more time to save, and so much more time to invest and earn more from your money.