I agree with others that you should find out how much she does know and if there is something special she would like to sew, 9 patch throw, straight skirt, large tote, etc are good beginner projects. I think teenagers are more willing to take part if the end result is something they can relate to!
Do have fun, I have taught several teens how to sew and they are fun to work with. (My sisters say I'm crazy!!)