I am not a parent, so a lot of times I don't feel qualified to talk about parenting concerns. I don't know what you're going through and I don't know what's right for your family. But I do know a lot about mental health in general and I hope that I can provide some resources if you don't know where to start.
The most important thing is to listen and be supportive. Knowing that they have an ally is extremely beneficial to someone who is struggling. Creating a calm, safe environment can help teens be able to communicate about what's going on in their life, without the risk of judgement or punishment. Consistent structure in daily or weekly routines also helps lift a burden of emotional work that they would have to do to figure out what to do each hour or day.
There's also a great program that NAMI runs called NAMI Basics, which is a free, 6-session education program for parents, guardians and other family who provide care for youth (age 22 or younger) who are experiencing mental health symptoms. While I've obviously not attended myself, the reviews are phenomenal. The next session in the St. Louis area starts in June and is located in Arnold.
Here are some additional resources that I highly recommend reading.