I need to hint my gf something im emberrased of?

me and my gf are both 13. her stomach turns me on a lot. a friend told me i should stroke it and tell her i loved her. i want to rly bad but im afraid 2. btw she dosent know. how do i tell her without sounding weird and how do i hint to her i want to stroke it and stuff? Im going to tak her to a movie soon and i want to tell her then... how do i get her to ask if i want to stroke it..