If you are going to perform oral sex on your girlfriend you will come in contact with her vaginal fluids. Usually, if a healthy woman, there is little or no taste to those fluids.
Just remember that vaginal fluids can transmit STD's so be sure that your partner is not infected before you perform oral sex on her.
If you want additional protection or prefer not to taste the fluids you can purchase something called a "dental dam" (it's used by dentists for some procedures--hence the name.) It's nothing more than a small sheet of very thin latex which you would place over your partner's vagina before you perform orally on her. A condom can also be used by unrolling it and cutting lengthwise and then placing the latex over her vagina. Another alternative is using plastic wrap.