Im 13, My 18 year old girlfriend making me ride her.?

Hi, please help! Im 13 and ive been dating this chick since i was 11. Shes 18 next month. Like ino it myt sound weird but we get on fair well. We know each other cuz she used to babysit me and shes friends with my older sister. Obviously my parents dont know about this, we kiss when they are gone out to drink for hours and hours. But now thats shes old, she wants to have with me and i dont know what to say because i think i love her. She told me id like it and id feel hot and good. And she wants to do it tomorrow night. What do I do because me nd the lads were lookin up the internet and it says 13yr old lads 's cant erect properly unless theyre very very horny and i duno what that is. Please help?!