How should I answer 'what's up' without sounding boring?

when a guy says 'hey what's up' after you greet them, and I'm talking about in person not over the computer. do they really expect you to answer? or is it just a simple greeting. I feel like if I say 'oh nothing' or something.. we'll have nothing to talk about. so do guys really excpect you to answer with what is going on in your life or at that moment. I really want to have a conversation with my crush, but if he asks what's up.. which most guys do.. I don't know how I'll answer or what I should say.
How should I answer 'what's up' without sounding boring?
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