How to achieve a flat stomach or small (feminine) abs?

I am 16 and there are 5 months left until summer.
I am 6′ and approx. 65kgs and I run 3.6km almost every day and eat healthily (the majority of the time).
I wish to have a flat stomach (or have small abs – feminine) so I look great in a bikini this coming summer.
Any tips or workouts to achieve this would be great! Thanks, x