Exercising and not exercising to loose weight?

I want to loose weight but do i have to exercise to?? or could i just not exercise and eat less?? I don't need to lose much just 5 kgs i lost 6 kgs but then i gained 3 kgs again T.T without exercising. If I need to exercise how many hours or minutes per day?? I want to work all my body especially the thighs arms and the stomach. that's basically all of it :( what do i eat and how many calories and nutrition etc do i eat per day??? im 13 and turning 14 and i look fat i have big bones that's y.