Assuming its mutual, you flirt with him, a bit obviously. If you have no evidence it's mutual, you flirt with him a little less obviously, and wait for a sign. And if it is definitely not mutual, you crush that crush and crush on his friend, or someone else, till the crush diminishes. Oh, wait... He's not a stepbrother, right?
You would just greet him normally and assume that there's nothing going on. Become his friend and hang around him more often like for instance, go for a walk with him or ask him to accompany you to a store. Your relationship will eventually grow and you will not have to tell him how you feel about him.
Well just greet him normally and try to send some gestures to show u r interested but don't come off desperate. If he responds then u r in and u can ask for his number. But start off as friends at first.