How do I support my depressed best friend?

What type of support your friend needs may vary a lot based on what exactly is going on. In general, I would be ready and willing to listen, but not insistent/pushy (don't force her to talk if she's not ready). Her willingness to talk and engage with people may come and go as her depression fluctuates. Also, if you know any good resources for her to connect with (a counselor at school, some place she could seek counseling with a parent's help, or even a helpful article or something you find online), feel free to share it with her. And of course, when she is feeling better and ready to engage with people again, be ready and accepting instead of holding the time apart against her.

One other thought - you mention you have feelings for this person, so another thing I would suggest is to put those aside for the moment, and focus on what she needs - not what you need (not saying you're doing this, but it can be easy to fall into this trap with relationships or people you'd like to be dating). Even if you were a couple, you couldn't be her sole source of support - it's not only not possible, it wouldn't be healthy. So, be ready to contribute to a group effort.

 

Agree with what Gravity wrote above, especially the last part. It takes a village, and whenever possible, involve someone with authority on the matter (school counselor, therapist, etc.) because they will be more objective than let's say, you, who are sort of involved in the situation. And don't forget to take care of yourself, too so you don't burn out. Self-care is important.