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.
When you are facing her quim, take a moment to study it. Note its shape, whether there is a horrible aroma, or whether it seems in disrepair in any way. If God help you there is a horrible aroma, breathe through your mouth. Remember that unless you give this a good college try, you will be jerking off tonight. And most importantly--watch the girl. She is excited enough to let you this far into her panties; check her out: this is what she looks like turned on. Notice little things: Are her eyes glazed? Is her breathing shallow and rapid? Are her nipples hard? Does she lie still, or rub her thighs together or move some other way? Does she whimper or make other noises? Pay attention to these things; these you watch for. Anything that increases those movements/signs, do more of that. If she slows up, change up.
Spread the lips and look inside. There will be another set of lips in there. Looking at the place where they come together in front, there will be a little button of flesh. This is the clitoris. If you push up (towards the head) on the mound above the girl's slit it will expose the clitoris. That makes it easy to find, and avoid if it is too sensitive. You might be able to arouse her by moving that mound up and down, rubbing the clit against its hood. If you mess with it directly she may scream and kick you. Instead, get a bunch of saliva on your tongue and lick around the clit, on the sides and the top. Watch her! Pay attention to her and you will notice the signs you saw of her excitement, either heightening or lowering. Experiment. Pay special attention to the places that bring on signs of arousal. Put your fingers in her pussy just a little bit and move it in and out. Increase the depth if she responds well. Soon you will taste her, and she may tense up. You will get the sense of something building in her, and pop! And you will be tapping that. Probably tonight.
It may happen that you can't figure out her cues and you don't know how to get her off. Ask her; if she is hot enough she'll point it out to you. You've learned something, and you get to watch her masturbate. Score! Go to work again, using more or less pressure depending on what noises she makes, parts she grinds, etc., and don't give up. Every woman has a magic way she needs to be touched, and it's different for each one. Talk to her; she'll soon realize that it is to her benefit that you learn to eat her pussy, and she'll tell you the kinds of things she likes down there.