Tweaser, perhaps we should change your name to "Teaser", because teasing always works for me in both giving and receiving.
First of all, have you communicated with her? Have you asked her what her preferences are regarding oral stimulation? She should be able to guide you. After all, every woman is different, and who would know what works best for her better than she does. Unless, she truly doesn't know herself, which is rare, but possible if she's young and inexperienced.
Well, for whatever it's worth, my advice is as follows:
Tease her! In other words, start gently kissing and licking the surrounding areas of her vagina, such as her inner thighs or above her pubic hair line, etc..., but EXCLUDE her vagina. The idea is to have her squirming before your mouth ever touches her vagina! Then leave that area and give her breasts some attention for a very short period of time, and then go back to focusing on the surrounding areas of her vagina (don't ever tease for too long because she'll lose interest). Keep it interesting at all times. Therefore, you can now give her an occasional gentle TAP OR TWO ON HER CLITORIS WITH YOUR TONGUE, which should start making her clit pulsate. Only give her one or two taps and that's it, and then go back to her inner thighs and tease her some more. Repeat that process, only next time you can give her a few more taps on her clit with your tongue. Then go back to the surrounding areas again. The idea is to have her squirming and already somewhat wet before you've even begun sucking on her clit! After tapping her clit a few more times with your tongue and then going back to some more brief teasing in the surrounding areas, it should be time for you to really get into it. Therefore, start gently sucking on her clit, and then leave it and go back to the surrounding areas for a second. By now, she should be squirming and wanting your tongue on her clit more than she ever has before, and her clit should be erect like a little penis and pulsating like crazy. If you've accomplished that, now you can really starting sucking on her clit, without anymore teasing, and finish your business.
Might I suggest, at that point, start fingering her at the same time that you're sucking on her clit. Start off with one finger, if that's successful, perhaps add another. But, again, every woman is different, so you may have to experiment with her until you find out exactly what works for her.
Well, I can't speak for your girlfriend, but I sure got myself excited. LOL
GOOD LUCK AND BON APPETITE!!!