Why can't i loose weight?!

It's all a matter of calories in versus calories out.
The ideal model would be to get three to four 30 minute exercise sessions in a week along with a calorie controlled diet of around 1500 calories a day (for a boy). However, following a calorie-controlled diet without exercise will work too (only slower, of course).

It is a lifestyle change. You will need to start reading the back of all your food packaging from now on. Gauge an idea of the amount of calories in the food you regularly eat. For example, a slice of bread, a bowl of cereal, a bowl of pasta etc.

As it's calorie-controlled, you will need to start maximising your "calorie-usage". Convert from:

Full fat to skimmed - full fat milk is 66 cals/100ml whereas skimmed is 36 cals/ml. You can cut down the calories you spend on milk for breakfast by almost a half.

Soft drinks to diet soft drinks - 500ml of a regular soft drink will have about 200-250 calories. Diet versions of the same drink will have between 1-5. The saving is huge and will allow you to eat something different instead (a bag of sweets for example).
The same goes for squash (no added sugar versions commonly hold a maximum of 5-30 calories per glass as opposed to the full-sugar versions of 100-200).

White bread to brown bread - Per slice, white bread has 120 calories in it. Brown bread has 80. A few slices of bread will now be a saving of 80 calories which can go towards something different.

A brown bread sandwich with a glass of diet coke will now save you 300 calories. Suddenly, you've got 300 calories to buy a dessert or maybe save up and have a large tea.
Just remember the 1500 calorie maximum and don't let it slip. Good luck.

Well usually just eating less and healthily can at the least drop a few pounds within a couple of weeks. If you're on about losing a lot of weight then just just eating healthy is not going to cut it, you're going to need to do a lot of exercise.
The main thing about losing weight is burning more calories in exercise than what you've eat in the day so you start burning the stored body fat. At the beginner's stage, that is a very hard thing to do because it requires Jogging/Running for up to at least an Hour straight, which is why in your case you must build up to it. 1500 calories a day is a healthy aim, but 1500 calories of healthy food obviously.
Diet wise you should stick to 1500 calories but try and change the diet around a bit so you have the same intake but different food so it does not get boring. Exercise wise, Anything Cardio will burn fat, so Jogging/Running, Swimming etc. that sort of exercise. Even walking around burns calories, but dont actually walk around for an hour as your daily exercise because that won't be enough calorie burning to make a difference. What I did is started jogging for about 15 minutes a day, then the next week work up to 20-25 minutes then the week after to about 40 minutes and so on but if joggings not your thing you can always look up other Cardio activities that seem fun for you, the key is to not let it get boring.(Any sports which involve alot of running are good such as football, tennis etc.) Try to avoid sugary drinks! Replace it with water, wther it be bottled, from the tap or even in flavoured squash (I was lucky enough to actualyl enjoy drinking water where as other's hate it, everyones different). But remember sometimes eating non healthy foods and drinks is fine as a little treat. Every week or so sometime me and my friends get together and order a take out and just chill back, having this in the evening wont suddenly increase your weight.
So remember, drink alot of water and when measuring your weight (which try to only do once a week) that a little increase in weight might just be 'Water weight'. Start of with a comfortable amount of exercise and build up your limit at your own pace. Whean eating, eat slowly and take about 10-15 chews on whatever is in your mouth, this way its easier for your body to burn. Take pictures of stomach once a month and try not to look at your stomach every day. I know it seem's hard when your getting changed and taking a shower but if you don't take notice of it untill the end of the month where you take a photo, you'll be able to see a difference alot easier :)
Losing alot of weight properly takes alot of dedication and no lazy attitude toawrds it helps. Eventually it will become a sort of 'un-boring routine' when you find all the exercise and food thats right for you.
I've been doing this for just about 2 month's and already I have lost a stone! good luck!

Weight loss advice and tips, and going through it myself.

try practising sports, start from little and then keep on going up and up on the sessions, you can start by walking 2 km everyday, and every week try to raise that number, after some time, when you feel able, why not give it a shot at running? or even bike or something like that
hope this helped
PS: it is not just about how healthy, unhealthy or amount of food you eat :)

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What is your height and weight?

Some people have an "optimum weight" where they can never lose any weight beyond that point.

bad snacks might be the problem