If you pass a girl in the hall, and you don't have time to talk, look them in the eye, smile, and say "Love your hair, Missy." Give them a compliment. Always look them in the eye if you are interested in dating them.
If you have time to talk, give them a hug (not too long or creepy) and talk about school classes, school activities, your life, your hobbies or hers, just make smalltalk. Google "how to smalltalk" to get more tips.
Girls will often look at cute guys, but that doesn't mean they want to date you. So ask their friends if the girl would want to date you. Their friend my have to ask the girl, then get back to you in a few days, so be patient. Or simply ask the girl directly. Ask if she has a boyfriend. "You're so nice you MUST have a boyfriend." Don't give compliments only based on their looks, they don't like that. They like compliments on their brains too. Focus on their strengths, not just their looks.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)