Hello,
I understand your concern.
Breast size depends on your genes.
You cannot increase your breast with any medications, allow it to grow naturally.
18 years is still a younger age, it would increase later, you need to have more patience.
Health Tips
You may follow certain exercise that focus on pectoralis muscles.
Have protein rich diet, like lentils, pulses, beans, soya products, etc.
Practice meditation regularly.
Stay positive and healthy.