Depends on your relationship with said girl. Are you guys friends? Starcrossed lovers? Acquaintances?
Just tell them, just tell the girl you love her simple as that. If she turns you down move on or try and earn her love (assuming this is a girl you love who you are not in a relationship with). But seriously whyy hesitate? What's holding you back? The only thing that should be able to stop you is you (Or the law >.>)
Were friends as of now. Not together. I try telling her all the time that I do love her but she doesn't believe me and I do everything I can to prove it but it never works.