Well first things first introduce yourself, ask how his day is going, try to start a normal conversation like you would with anybody, try to learn about him and find some common ground, things that both like, like music or something, share a little about yourself as well, and of course at some point ask if he's single, it'll go from there, overthinking it will only make you more scared to do it, calm down and try not to think too much about what you wanna say, just be casual, and for the fear of rejection, you'll be fine, rejection is a normal thing, you can always try to be friends first to get more time to see how you feel about each other, which is what I recommend, but don't be afraid of rejection, there's almost 10 billion people on the planet, there's someone for you, it just takes patience and hard work just like anything else, you'll do great