For the serious question with all the not-so-serious answers from your friends:
Here's my chance, she likes me and she wants to fuck me. The foreplay is hot and heavy, there's been some mouth-to-cock contact and she's obviously as big a fan as me. Really obviously; like here's my face, here's her pussy, and I can feel it radiating lust against my cheek. It beckons me to rock her world and make her never forget me--
And because I have no clue, I make her sorry she decided on sex tonight instead of the late show and I crawl home clutching the remains of my erection and my pride. Well, maybe not that bad, but bad enough. And we know that's pretty much the end of the relationship because we could never look at each other the same way again.
This is something that has happened to most men at least once. Some decide right there that they will never be among the muff-diving set, and others are naturals and they get their face wet with hot girl. Most of the world pokes and prods and wonders if this beast is going to wrap a tentacle around its neck and suck onto its face. Everybody likes pussy, right? C'mon, girls, you like pussy because you have one and we don't. But even if you adore it later, the first time you have one in your face can be terrifying.
Some people study ahead of time. The Anatomy Coloring Book is a great place to start as a kid, followed by real anatomy books, Shere Hite, porno, kissing feeling groping and finally *fanfare*-- a real pussy in your face, and all your knowledge and preparation goes flat beside the road. Like an armadillo.
Not to worry. Eating pussy is easy if you remember three words:
Watch the girl.
By the time you've gotten to the point that she (your wife and or monogamous partner) has her panties around one ankle and her thighs on your shoulders, it's pretty much a given you'll get laid. But there are tests you must pass, young Padawan--and if you fail them, you might not even be getting a ride home from the party.
She's most likely very excited already. If she's been sucking your cock or touching you or herself, she may come in your face when you begin. Be prepared for this, because if you are doing anything right at all she will come in your face anyway. That's the goal. Your face will get wet, your hair might get wet. You may need a new shirt, depending on the girl, and you won't know if she's really wet or not until the moment of triumph, so if you drove keep a change of clothes in your car.