You know what? Since you don't know what to do, I'd reverse his question and ask HIM to tell you his first since this was his idea and stick with that even if he says - "no you go first!" Then that'll give you a clue as to what he's thinking about and how intimate or crazy his stuff is that goes through his head, or if it's romantic things he'd like doing with you. If his fantasies are too wild for you, just tell him like, "well I've never had those kind of fantasies but I have thought about. and then just tell him a few thoughts you've had and tell him that's about it. He might be looking for you to tell him crazy sex things you'd like doing with him or actually just nice sweet romantic places you'd like to do where you guys could be alone and do whatever you want. Maybe you've wanted him to oral on you but he hasn't so that could be one you'd tell him. Or possibly you'd like him to do you doggy style or whatever ways you've not had sex yet with him or other sexual things you'd like doing. I'm sure that once you hear his it'll give you some good ideas of your own and you won't be just blurting out stuff that. doesn't turn him on.
If you have any, tell him. If you don't have any then be truthful and simply say so but you can also say that you are hoping that he can awaken the sexual dreams within you.
No need to invent any stuff in order to keep up with perceived standards or to think that there is something not alright if you don't have any as of yet. They will come later as you become more comfortable with your sexuality.
Well I won't give you ideas per say, but I will give you stuff to think about that would help. Is there a romantic movie that you love? you might want to re-enact a love scene from there. Do you have any passions? Like are you a musician or a painter or anything like that? you might want to make up a scenario including that. don't feel too constrained, this is a fantasy after all. Anything could happen.
The best advice here is to stop and thing about it. If somebody else gives you ideas, they may be things you are uncomfortable with or simply wouldn't be fun for you. Think it over, you'll come up with something.
Well don't just make something up. Just think about different scenario's in your head, things and situations that you would like. Think back to what you think about if you masturbate. Ask him the same question and bounce ideas of each other.