It's an old saying, boys, but it's a good one. Be yourself, and be honest about yourself.
I'm not saying you have to pour out your soul to her right away. I'm also not telling you to be brutally honest. We women are fragile, fickle creatures and can be hard to handle, hard to judge, even hard to tolerate sometimes. But we always appreciate honesty.
If you're not ready to talk about something, just say so. If you need to tell your girl something, make sure she understands how important it is. Don't deliberately keep secrets and, although I hope such a thing doesn't happen, be honest if you've done something wrong.
No matter how terrible it is, if you've done something and you know it'll hurt her feelings, you should tell her. Never let her find out from anybody but you if you've got something festering inside you or you know you shouldn't keep it from her.
If you think something she does is cute, be it her laugh, the way she picks at food, holds your hand, does her hair or whatever comes to mind, you should say so. Don't keep it to yourself. Girls love to hear what you like about them.
Most of all, don't be afraid to be honest with her. If you are as important to her as she is to you, she will appreciate your honesty.