It is not always easy to know how to deal with a depressed teenager. Teenagers have a lot of hormonal changes going on, so they might be naturally moody. However, when teenagers have the illness of depression, they are suffering from a chemical imbalance.
The best way on how to deal with a depressed teenager is to take the teen to the doctor. The teenager needs to be properly diagnosed in order to receive treatment. Often, the best way to start is to go to the family physician, and then to ask for a referral to a psychiatrist if the physician thinks that there might be a mental health issue at play.
Often the teenager doesn’t understand mental health, and will be scared to go to the doctor. Talk to the teen and tell the teen that untreated mental illness gets worse with age. Ask what type of life the teen would like to have, and talk to them about how untreated mental illness can get in the way of that. It is better to be behind one year or semester than live a decade of ruined opportunities due to mental health concerns.