How to approach someone?

Last week at the bus stop at my university I saw the most beautiful boy, but I only saw him through the window after I got on the bus, so I couldn't talk to him.

I was wondering, what if I see him again? What should I tell him? Is it even a good idea to talk to him at all?

He seemed very nice though. I saw an elderly lady ask him something and he seemed very kind though the window, but idk haha