A girl you like.
The girl you often meet.
Be kind to everyone, not just her. You don't have to go out of your way to be nice to everyone else, but do try to be polite and considerate in general. Acting arrogant and bullying to everyone but her will make your kindness seem fake and insincere. Instead show her that you're a high-quality person overall, and she'll notice that you take extra steps to do kind things for her specifically.
Build a foundation of little gestures. Being nice to a girl doesn't have to mean presenting her with a huge gift or making an grand gesture; in fact, doing that at first could be overkill. Start out with small, simple actions that show her you care about her. Greet her with a smile and a friendly hello every time you meet. Open doors for her, or offer to carry cumbersome packages. If you have to walk across the classroom to pick up a handout, grab one for her, too. These little everyday gestures will increase the trust between you, and telegraph that you take the time to think about her needs.
Stay humble. Most people like to talk about themselves, because it's a topic they know well. When you're talking to her, focus on her strengths and interests. She'll start to feel comfortable sharing parts of her life with you, and you'll avoid looking arrogant or self-involved.
Make her feel special. Communicate your interest by finding subtle ways to let her know she's special to you. Flirt, and pay her a genuine compliment, like 'You have the most beautiful eyes I have ever seen.' When you're in a large group together, make lots of eye contact with her and consider throwing out a subtle wink. Tone down your interactions with other girls when she's around. These actions will let her know, in an easy and comfortable way, that you're interested.
Be there for her on bad days. If she's having a tough time, or simply needs to vent about something that's bothering her, be a compassionate listener. Let her talk without interruption, and let her know that you empathize with her and care about her feelings. Don't immediately jump to offering solutions; if she asks for help, then you can start collaborating on an answer.
Make bigger gestures when it's appropriate. If she's been receptive to your actions so far, then she's probably comfortable with you. Start making bolder gestures of kindness and interest - offer to walk her home or to class, send her cute texts or IMs, and start getting to know her friends. If her birthday or Valentine's Day is coming up, arrange for her to receive a small, tasteful bouquet or a single red rose tied with a ribbon. Try to take pleasure in her happiness instead of anticipating what she'll do to 'pay you back,' and keep these actions sincerely motivated.
Be considerate. Above all, take care to consider her thoughts and feelings. Showing her that you can do this accurately and dependably will help your connection flourish.
Be nice and gently.
Show her how you feel.