Well I'm twelve years old and I need to loss 20-50 pounds to have a flat tummy at the beach in 5 weeks so I'm really stuck it was a last minute thing and I don't know what to do it is probably virtually impossible

It is neither safe nor realistic to attempt to lose that amount of weight in such a short space of time.

You can't lose that amount of weight. What you can do is what bodybuilders call Stomach Vacuums. This is a great exercise for making your tummy look flatter.

Stand tall, then suck your tummy in as tight as possible, as if you were trying to touch your belly button to your spine. Hold for ten seconds, working up to 30, then a minute. You'll find that when you are doing it, you have to breath in the top of your chest. It's tough, and you'll feel sore the next day, but if you do it regularly, it will tighten up your whole mid-section. And it gives you great practise at sucking in when a cute guy comes along.

While you are working on your abs, five pounds is a more reasonable goal and well worth doing. You will need to figure out how many calories you need each day to support your current weight. For example, here's a place that will calculate that. Adult Energy Needs and BMI Calculator (http://www.bcm.edu/cnrc/caloriesneed.htm)

Let's say you need to eat 2000 calories per day. A pound is equal to 3500 calories. So to lose 5 pounds, or a pound a week, you would need to NOT eat 3500 calories per week. That is you'd have to short yourself 500 calories per day and only eat 1500 calories per day instead of 2000.
(2000 - 500 = 1500. I know it's obvious, but... )

It easier to do that if you eat lots of foods that are high in fiber--mainly fruits and vegetables. Fiber makes you feel full and also slows the absorption of sugar, which is a good thing. Protein and fat also slow down the absorption of sugar and prevent blood sugar spikes. Try not to drink sodas, fruit drinks, or even pure juice, as these all mess with your blood sugar and sense of hunger. Even diet sodas are bad. Just drink water, fruits and vegetables, whole grains, and of course protein and some fat. Beans are good, low fat meats, low fat milk, etc. If you make yourself a sandwich, use whole wheat bread, minimal mayonnaise and LOTS of lettuce, tomato, and pickles or peppers.

But count your calories!
There are lots of apps and places online that can help you do this.
Good luck!

PS. Don't try to lose 20 pounds in 5 weeks. You will make yourself sick. Also, you are probably still growing, so not eating enough could affect your permanent height. Instead, pace your eating so that you are always a LITTLE hungry, but not starving. Make sure your diet is very nutritious. It would be a good time to take a daily multivitamin.