You say "Hey" at the beginning of a sentence in casual English when you want to get someone's attention. For example:
Hey, Hitomi, can you hand me that box over there?
It's common to say "Hey" before you ask someone a question, or when someone has done something that's wrong or unfair:
Use "hey" with people you know or are familiar with. When you're talking to people you don't know as well, "Excuse me" is more polite.