First convo with a guy need help.

So I am interested in a co worker. I know most people say bad idea! But we don’t directly work with each other I’m in my office he’s in his. Interact sometimes. I have a feeling he’s interested but still not sure. I have never made the first move and I don’t want it to be awkward if he isn’t interested or anything lol I asked him to be friends of Facebook and he accepted. And I want to send him a first message. Any ideas? I have always been the one guys approach never the approacher lol he’s a little shy! But first message convo starter ideas would be great! Lol also how do you know if someone is interested or not? Lol I’ve been out of the dating game a long time and ready to be back in! Thank you!