Hi dear,
Being a friend to your teenager sounds easy but is quite difficult. You need to tolerate some things and be patient always. If you do not understand what to do just start asking your child of how was their day, share jests, give them reassuring smiles, tell them about your childhood, you life, your phases as a teenager so as to win their trust because friends should feel comfortable whatever they talk about but be form at points where you should be.
Hope this answer your question.