What do you reply to 'sup' as in what's up ?

This guy at school ( who is really popular ) said 'sup' to me what do I say I just groaned I mean that was the stupidest thing I could have ever done I mean seriously can you Imagina that ' sup' and then someone groans but he didn't seem to care STILL what am I meant to say? He is obviously embarred to like me since when he is with his friends he only sneaks a peak at me and when we are on our own he flirts like hell and is really nice to me so back to the question now that you know what we are like what do I reply to 'sup'? Thank you