You cannot lose fat on a certain area of your body - you'll have to burn it all over. Ab exercises won't really help with that - best consequence is that everything just looks a looks a little more toned, worst is that by bulking up on abs, they come out and make the belly look even bigger.
So aim for burning more than you consume and depending on the amount of body fat, you'll see a flat tummy within some weeks or months.
Everyone is different. And some studies indicate you can lose fat on each body part.
Careful eating habits. Cut processed foods & added sugar. Cardio & weight training.
Losing belly fat is just a matter of losing body fat, which comes from burning more calories than you consume, simple as that. How people recommend you go about doing that simple process has become a multi billion dollar industry filled with misinformation & scams.
I say, more vegetables & water, (and decent protein) less processed foods & sugars.
More exercise of any kind. Most people think cardio when losing fat but including a moderate amount of weight training offers numerous benefits to the fat loss process.