if you have paypal you can use that to buy from someone in the user services section.
Otherwise check out circle pay (I can't use that as it doesn't work in Canada so I don't know if you can use it under 18)
or Coinbase (I recommend this one)
or Kraken
or Local bitcoins (if you don't mind meeting people in person)
or paxful. (Useful but you'll have to read a guide on how to get btc through there)