swelling and irritation of the vulva (the folds of skin outside the vagina)
pain or burning when peeing or having sex
thick, white vaginal discharge that looks a bit like cottage cheese
Some girls will have several of these symptoms; others may only notice one or two.
It's best to see a doctor if you think you may have a yeast infection, or if you have anything different going on, like changes in your vaginal discharge. Other infections can cause similar symptoms, including some sexually transmitted diseases (STDs).
Your doctor can recommend the best treatment for you. If you don't have a family doctor, you can visit a health clinic like Planned Parenthood. If your school has a student health center, you might be able to get checked out there.