Need help with a girl

Im in year 12 studying my A levels and I met this girl. I just cannot shake the feeling that she is perfect. I have had a few conversations with her online and one or two within school, but we're not in any of the same lessons and I rarely see her around in school. I'm happy to admit it, I'm terrible with girls - its not a lack of confidence its just I don't know what to do. Any advice? Should I just ask her out on a date or is that just weird given that I dont really know her very well? Should I try and message her more? Or just try and make the most of the opportunity when I do speak to her for real?