Girls would want to hear how much love you have for them, the vision the guy has towards future for the both of them and what he can do for the girl for love. Girls also like a guy with a good thinking capacity and being so responsible and mature in the head.
What you like most about her. Pointing out her positive qualities, and always being honest with her. She'll want to know that she is the only one you think about if thats the kind of a relationship you are in.
That you will always be there for them good times and bad. That you'll be her shoulder on cry on and if anyone messes with he you'll beat them up. Also that you'll never leave her and when you say "I love you", you mean it.