How should I write?

Last month, I went to some high-end restaurant with my family. (It's the usual restaurant that I always go to.)
I saw a gentlemen staring at me, so I returned a smile.
Today, I went to the restaurant again, and the waitress gave me a card telling me that
the gentlemen gave me his number and he wants to ask me out for dinner.

I am interested in having a conversation with him...
but don't know what to write...as it's the first time experience for me.
What should I write? or should I just leave it...?

So he knows you go there often? How would he know that? Be careful who you meet. I guess you could write back a time to meet for a meal. Do it during the day, so when you leave it is still daylight out. You never can be too safe. But you don't have to write a letter back. Just something simple.

I wouldn't meet for dinner for the first time.

I recommend meeting someplace busy and highly-trafficked in broad daylight for the first time - like coffee in a mall or something. And be very cautious.

Yeah that's what I meant to say too. I probably should have phrased it better. Meet him during the day at another place that's busy. He might already know you go to this restaurant often and meeting him there would just reinforce that into his head. Make sure someone else knows you are going there to meet him. Like I said before about writing him back, just keep it simple. Write a place and time. Maybe make it a day thats far enough ahead so that he can meet you there incase he has plans. If he ends up not being able to meet you at that time, he can leave another note at the restaurant. He will do that if he is a gentleman. Or just leave the note asking him if that day and time is okay and he can write you back again. I don't know how often you go to that restaurant so this might take awhile. But it's better to keep him waiting :)

I guess the owner told him that I said "sure I'd love to have a lunch with him."
I will be careful. Thank you for your comment and concern.