Eat healthy food, fruits, vegetables, in special green salads, drink 2L of water every day. For girls: 2000 cal daily and for boys: 2500 cal daily. Also I advice you to take your first meal at 9-10 AM and the last one at 6-7 PM.
I assume you also don't want liposuction or other such surgical methods, either?
In that case, it all comes down to exercising and starving yourself. Burn more calories than you take in while barely eating enough to not die of malnutrition. That's it.