I usually struggle to eat 1200 calories a day and today I've ate about 6000,what will I gain?

there are about 3500 calories in one pound of fat which means you will keep gaining about 1.5 pounds per day if you keep eating that much.

Losing weight is not just about how many calories you eat, but the type of food you eat is even more important, more than most people realize. instead of eating foods like chips breads candies and regular sodas or juices you must switch to diet drinks, low carb/low calorie foods that are high in protein, fibers, antioxidants and minerals etc.

Foods such as fruits, vegetables, meats, even eggs. You should lay low on the fruits and breads because they contain many carbs and sugars but you can have some fruits. I highly recommend that you try the South Beach Diet. A few of my friends have seen very good results.

I'm sorry for the long read, but I hope it helps. You don't want to wait to lose weight, it will be difficult at first and you will be hungry but keep reminding yourself about the prize, which is a healthier, better looking body.