I'm 19 years old and my face is very round and fat. How do I lose facial fat, and make my face slimmer? (photo)

Achieving a chiseled, less chubby face requires to address multiple factors in the face and the body composition to contribute to this physical finding. This analysis will help to determine what is the plastic surgery procedure(s) what could best help to correct a chubby face, thus achieve a slimmer more chilled face. 
Some of the physical findings that may need to be addressed to achieve a slimmer / chiseled face are: 
Lower anterior cheek fullness: this could be addressed surgically with buccal fat removal 
Lower posterior cheek fullness: this could be achieving my treating the masseter muscle ( a masticatory muscle) with Botox. The masseter muscle gives bulk to the lower posterior cheek fullness at the level of the angle of the jaw bone 
Overall body fat: weight loss to achieve a normal or low normal weight for the height ( normal BMI) is a must. (most models with chiseled face have  are "skinny" and therefore have a BMI in the low normal range) 
Anterior and lateral cheek bone projection: lack of cheek bone projection won't allow to have a chiseled appearance, event if the above factors are achieved. Therefore, it someone has not good cheek bone projection, he or she may require cheek augmentation/ enhancement with either soft tissue fillers like Voluma or cheek implants 
Saggy facial skin: young people usually doesn't have saggy facial skin. Patients in the 30s or 40s may have some aging related sagginess that may be improved with soft tissue fillers 
Patients interested in determining if buccal fat  removal may help to achieve a chiseled face should make a consultation with a Board Certified Plastic Surgeon specialized in plastic surgery procedures of the face.

You are a good candidate for the buccal fat pad removal, it would slim down your face as well as Botox to the masseter muscle.