how to predict an orgasm !!

I never had an orgasm before... I recently started having sex with my bf but I've never achieved an orgasm.. nor when I masturbate. My friend described the feeling before orgasm as a slight tingling. Another girl told me it feels as though I have to pee. When I have sex with my bf especially in backshot I often feel as tho I need to pee so I tell him to slow down. If I allow him to continue or go faster do you think I will have an orgasm or is it really pee that I feel?