1) Get a bank account
2) create a wallet in a secure way (learn how to do it properly and safely, this is the single most important thing)
3) Go to local bitcoins or whatever your country's equivalent is. Buy bitcoin with your bank account.
4) Use decentralised exchanges, like Waves, or anonimous ones, like ETHdelta. Do not go to exchanges, which require verification, and always be on the lookout for it, because, as underage, you're screwed, once they ask you for ID, they'll keep your money.
And NO, doing cash for Bitcoin transaction IS a bad idea.
You can find a way to do it.
Good luck!
Why did you not do it through localbitcoins, paying by the bank.
Man, you could have gotten into much more trouble, so just be happy that you didn't get hurt, money will come.
I feel sorry for you.