The big O?

Hey ladies! So I’m a 19 year old virgin and have been waiting for the right time and person to have sex with. Me and my current partner are making plans to take each other's virginity, however, he asked if I’d ever orgasmed before. I was honest and I told him I didn’t know. I masturbate and it’s been-as eve would say- “mind-blowing” but never knew if I reached that point orgasming or not.. so what I’m asking is, what does the big O feel like to you? I know it’s different for everyone but what is a telltale sign that I’m reaching that point?