@Lis Cutting out sugar is a good start, it’ll help with both weight and your skin (imrpove complexion). Try to eat more fiber (like vegetables, fruits, and whole grains) and drink plenty of water. Even light workouts like walking, jogging, or short home workouts can help burn calories and improve your mood.
I know you want to look your best, and I get that pictures last forever, but please remember your quinceañera is about celebrating you so try not to worry too much about your weight, focus on feeling confident and healthy so you can enjoy it to the fullest.