Hello. doctor i am 22 year old female, weight 37kgs, height 4'9. i have a flat chest. very small cup size. how to increase my breast size?

Unfortunately there: Aren't many good means besides surgery, "push up" bra's and weight gain(though this causes fat to be deposited everywhere, not just your breasts) in regards to increasing breast size. Try to avoid becoming "consumed" by an issue like this and realize that you are much more than just the "superficial" aspects like appearance.

Breast augmentation: Breast augmentation is the only way to add size to your breast alone. You are below normal weight and weight gain will add size to your bust line. If gaining weight does not add to your breast size then augmentation is an option,

Your cup size: ...is less important than your(hip:waist:shoulder:breast) proportionality. You may already look perfect. If "not"-1) standard augmentation with contoured, cohesive gel implants. With the numbers you mention, I would be very reluctant to use anything much bigger than the smallest devices available but the device diameter is more key than is the volume 2) Fat transfer may be just the ticket for you.