The girl I like told me that she was unhappy. What should I do, how to comfort her?

A girl can tell you what she is unhappy about, which means that she has you in her heart and probably likes you. If you can comfort her, you can become a good listener. When a girl tells you something, she doesn't necessarily want you to comfort her. It's just that she wants to find someone who can convince her to speak her mind. As long as you two are talking and listening and you like her, you must tell her.