Hey, as you know already from the title, I'm new to Bitcoin, I would like you to help me by telling, where to buy, when to buy, where to sell, when to sell, things like that.
Important: I'm 17 years old, some methods may not work if I don't have 18, and that's what I have noticed.
I hope you can help me get started.

The first thing to do is to create a bitcoin wallet. You can open a blockchain wallet via blockchain.info .
Since you are still new to bitcoin, I'd wouldn't advise to go into trading straightaway. I'd suggest you participate in signature campaigns to earn bitcoins. Thereafter, you can use your earned bitcoins for trading.