Join group conversations that she's in, listen when she talks and throw in an occasional "wow" or "that's cool" or whatever appropriate phrase comes to mind (to show her you are really listening to her). Don't fawn over her or be fakey. Be a nice guy and good listener to other kids too, both boys and girls, so you get a good reputation as a friendly, supportive guy.
Do you dress like the other guys? (One fellow in my 6th grade buttoned his shirts right up to the top so he looked very formal, but the other guys left the top button open and were more casual-looking.) Be sure to take a bath or shower every evening or morning before school, brush your teeth (and go to the dentist twice a year), comb your hair, and look like you care about your appearance.
How does that sound?