I have no idea what to say to her when she says it. I usually say stuff like, thank you, thats sweet or mhmm hmm lol
That was me in my last relationship lol. It is hard to know what to say.. I'd say yeaaahhh or mhmm.. Sometimes I'd have to pull out I love everything I know about you.
You are right not to say it if you don't mean it though.
sometimes she says "im starting to have strong feelings for you" or "i really like you"
Haha that is cute. She does sound young, but really excited to be with you.