I am a girl and normally don't have to much acne on my face, but they show up more when i am on my period. how can i totally prevent and cure them?

You can't: Acne is a frustrating condition that can be treated, but rarely prevented 100%. There are multiple topical medications like benzoyl peroxide, clindamycin, retina-a, and adalapene that due a good job of treating outbreaks.

Can reduce...: Menstrual acne occurs in many women. It is due to the Progesterone rise (mid-cycle) causing increased sebum, and to the Estrogen and Progesterone decline prior to bleeding--causing relative predominance of a woman's Testosterone. Birth control pills (estrogen) can help counter the effect of Testosterone, thus reducing acne. Spironolactone can reduce Testosterone production, also reducing acne.