Extreme workout?? Personal trainer?

Ok so i'm about 10 pounds overweight so i am going to have 3 sessions with a personal trainer. I want him to teach me how to work out on my core. Approximately, if i worked out say 3-4 times a week going hard on my core, how long would it take to get a slim belly? I'm 14, 5' 7 and have chub for a waistline and stomach. Can anyone help me out??!!