I personally don't get off at all by receiving oral sex. But I'm sure some girls do, and it can be a turn on if done correctly :) Make sure to pay constant attention to the clitoris. I can't speak for other girls, but fast tongue motions are too much for me! I like slow steady pressure from a flat tongue. Also, I would rather have a guy's finger inside me than his tongue. It is longer and more firm. But make sure your fingernails are trimmed! I like a quick, upward scooping motion. But like I said, all girls like different things, so a lot of it is trial and error. Just remember, she is probably nervous about you going south too, so just take control and she will love it :)
don't used the same stroke over and over again. mix it up. go in, play with her clit. most guys seem to think that it just the vag opening that's sensitive but it's the whole thing. a random love bite on my thigh drives me crazy. you can use your hands squeeze her butt, thighs or finger her while licking the clit. (nice)
but DO NOT do this if it makes you feel uncomfortable.
As a master Cunnilingust here is what I recommend. Stick with the tongue. Find her clit with it. use your hands to keep her lips spread wide open or ask her to keep it spread for you. A lot of women like doing that for their man. Remember the clit is VERY sensitive. DO NOT ATTACK IT ! flick and play with it very very gently. Listen to HER ! ! If her breathing is increasing and moaning your doing something right. Suck on the clit very gently... As her how it feels. don't be afraid. Every woman is different. Role play with it... Play that your a virgin and she's a street walker. MAKE HER tell you every single move to make. How to lick you etc. I wouldn't worry about fingers etc right off the bat.. Relax and enjoy... When she orgasms and your giving her oral you will understand the entire pleasure of the experience.
At the very top of the outside of the vagina is a flap of skin known as the clitoral hood. Under that hood is the clitoris. If you use your tongue just on and around this area to start, then lift the hood and lightly flutter your tongue on her clit, you should end up with a very happy women. Do a image search on the clitoris to get a better idea of what I'm talking about.