hello fellow lgbtq+ person!
Maybe figure out if he likes boys too first before you try anything. (I’m assuming you are a boy but if I’m misgendering you please tell me!) If your certain he does then I would just try to get close to him. Try and become friends? Going up and admitting your crushing on him might be a little ambitious if you don’t know him.
I would just try to start a conversation or small talk with him. Just a little “hello” or a question or two. If you guys get to a good point you could admit it. Take your time though, don’t try and rush it, that could get a little messy.
I’d just casually bring it up in a conversation but that doesn’t always end up how people want it to be.
I hope I was helpful, I’m not great at advice 
