I know this question is so cliche, but it's driving me nuts.

I've never felt this way about a guy before, literally speechless and shy when I'm normally outgoing, and he's my coworker. We've never quite had a conversation before aside from work-related issues, but I asked him to dinner with a lady coworker and me tomorrow, and he told me it would depend on how he feels after work since he's always really busy going to different school sites. He said if not tomorrow, he'll find another day.

He's super laid back and I'm super laid back, but what do we talk about? I feel dumb for asking, but how do I find starting ground without it sounding obvious I'm trying to flirt?