Different girls like different things. Many girls love being told their beautiful. They will always say they are not, but trust me... Every girl does it! Some girls love being told they are smart. And girls like me, love to be told "YOU TOTALLY HAVE A CHANCE WITH ANDREW GARFIELD!"
Complimenting her appearance always works, but if it is something more personal, it will mean more. If she talks about being insecure about her hair or something, run your fingers through her hair and tell her how much you love it. Or if she has a poster of her favorite band, compliment her taste in music. Tell her something you love about her, like how she is always there for you or has a great sense of humor. There are no real guidelines, just make sure you are honest.
Being honest to her. If you like her eyes then you should tell her that you think her eyes are pretty. But don't make up stuff. You have to honestly think it. That way it's sincere and she will feel special. :)
Um if I were you, I would compliment her on how pretty she is and how her personality is even more beautiful! Then I would say how happy I am to have her in my life. Then the cherry on the top thing to say is that my life wouldn't be as fun and awesome if I didn't have you. If a boy said that to me it would be so beautiful and awesome! (:
Tell her she's beautiful. Once you do she probably will say she doesn't believe you and other things like that, But you would not believe how much of a lie that is. Girls love to hear people call them beautiful. Don't call her cute or pretty because "cute" is like your talking to your pet and "pretty" is like the background it makes a girl feel like she is just in the distance and that your not close enough. It'll make her feel unaccepted.
DO NOT CALL HER HOT!!!! Girls hate when they get called hot they love it when we get called beautiful and just listen to her to a girl that's a big complement and it sounds creepy but when a guy stares at a girl and she sees him than he smiles and looks away that's a HUGE complement and laugh at her jokes and tease her it's probably the best form of flattery hope this helped :)