If you think they would immediately disagree with you no matter how you ask, I would suggest that you wait a few years then ask. Asking your parents if they will let you ask somebody out would be...
say this to her,"Hey, I just remembered that I don't have work tonight. Well, I don't really have anything to do.. Want to go to (u say where u want to go) with me tonight?" it will work, i am 99.9%...