What to do if i'm a 15 year old girl with autism and i think i'm showing signs of depression?

Feeling Good: There are several types of depression. Reactive depression occurs as a result of loss and eventually we adjust. If one does not adjust or if the cause is not clear certainly seek professional help. "feeling good" by david burns is an excellent self-help book based on science. Don't let the thickness of it intimidate you. Take it bit by bit. The very best to you.