Flirt. Get her number. More flirting. Talk about sexual stuff during text as it's less cringing than real life.
Ask her out on a date casually, then if that goes well, ask her out on a real date and make sure she knows it's a date.
However gathering from your age, experience and the girl, I don't think she's interested. You have a small window to act on, don't delay.
Keep us posted