When you look at the above photo, where is your eye instantly drawn to? More than likely the teen on the left. Why? Because she’s the largest thing in the photo.
I emphasized her by moving closer in and by having her stand away from her friends—thus making her look bigger. Of course, other principles you’ve learnt so far also helped draw your attention to her. Her friends are slightly out of focus, they’re looking at her, and she’s standing to the side (rule of thirds).