how can i say i love u to my girl friend

Don't express your love in words. This would not sound sweet. Express your heart in your bahavior and companionship with her without any aggression & hurry. Do love in confidence and not with a doubt in mind. Don't react or express with hard attitudes to her statements, likings, behavior & nature. This will turn you both in a loving and all time couple.

Why don't you start by saying 'I love u' but in a different language. And if you have said a few times to her before both of you go home, one day she'll wonder the words that you are saying. then she'll go and find out herself or simply ask you. then you'll be able to express your feelings indirectly to her. Hope it'll work for you. Best of luck!