Look into her eyes. Make eye contact with your crush to show her you are interested in her. If she looks your way, don't take your eyes off her.
Smile at your crush. While you're holding her gaze, give her your sweetest, most sincere smile. This will show her you really like her.
Hand her a flower. If you see your crush in school, or at work, leave a flower for her on her desk, or hand it to her without saying a word. She'll know you're interested in her as more than a friend. Consider presenting her with a rose. One single rose means "I love you."
Serenade her with music, if you can play a musical instrument. If you know where she lives, standing outside her bedroom window playing music is a very romantic way of telling a girl you like her. She will have no doubts.
Touch your crush, if you get the chance to be close to her. Putting your arm around her if you are both at an event together or holding her hand will convey your desire to be closer to her. Pay attention to how she reacts; if she pulls away, that means she's not interested in you in a romantic way.