Do you want to date her? Then ask her out. She says yes, or she says no. Tell her you've come to think well of her. She believes you, or not. If she says no, it's not a big deal. You seem to think you will be annihilated if she doesn't respond to you. You won't. You'll just move on.
The only reason to wait until she's 18 is if you want to have sex with her. Until then, you would be guilty of a crime, called "having sex with a minor". Besides, you're both way too young for a serious relationship to be wise. You have many years of studies ahead of you. Date casually, have fun, enjoy growing up. I know you think you're a man of the world now that you're out of high school, but you're still 10 years below the median age for getting married. There's no reason to hurry and every reason not to.
For what it's worth, I got married at 19 and it was the dumbest thing by far that I ever did. Fifteen of the best years of my life down the toilet. Don't go there.