First of all, I should say that my experience was that of an eighteen-year-old girl with a much older man, which makes my experience different than others and specific to me, but I think my advice will be helpful to anyone considering having oral sex for the first time.
The first time I gave a blowjob, I felt like a rock star, a porn star, and a princess all rolled into one. Although it was my first time, I had practiced and studied for it like an SAT. I had read Oral Sex For Couples - The Art of Cunnilingus and Fellatio and Guide to Licking and Sucking - How to Impress Him with the Best BlowJob - The Best Illustrated Guide to Oral Sex. I took notes. I made index cards. I copied diagrams and committed them to memory. If I could get a five on the AP Literature test, it had seemed to me, I could teach myself to suck a cock.
Along with my book learning, I had also created "labs" for myself. I had tentatively explored with bananas and cucumbers, mimicking the diagrams in the books, and watching myself lick and suckle them in the bathroom mirror. I had touched the back of my throat with the handle of my toothbrush every single night for two months to diminish my gag reflex. At first, I nearly retched, but I made slow but steady progress, and it got easier and easier, but I had to be committed, tapping my tonsils with the intensity and single-minded focus an Olympic gymnast practicing backflips. After sixty straight days, I had been able to drop the whole length of the handle down my throat, and I felt I was ready to swallow swords.