Advice for my first time toking?

Ive never smoked pot before, even though 95% of my friends have already. I was thinking of buying a dimebag to enjoy for the first time on 4/20 before school. What should I expect? How many drags would I have to take off a joint on an empty stomach to get high? Please don't chastise me about pot, I just want to try it at least once.