Tips on how to start a conversation with my crush

I have a crush/ infatuation with a guy (m26) who I see at my workplace. We don’t directly work together but in the same building. I (f25) see him as he comes into work.

Last time, I opened the door for him, he thanked me and I was so nervous that I froze and ignored him.

Any tips on things I could say to him in passing? or ways to approach him. I don’t often have crushes so I really want to make an effort to get to know him. Also, I’ve only had 1 date my whole life so highly inexperienced.