Talk to your doctor. There are often medical reasons people gain weight rapidly.
(e.g. Thyroid or hormonal)

Exercise is important. At least half an hour every day. Walk or cycle to work.
Eat healthy.

Foods to avoid:

Dairy foods. (Butter, Cheese, Milk)
Eliminate sugar and Alcohol from your diet. (Also avoid artificial sweetners)
Fried foods. (Potato chips & fries).
Processed carbohydrates. (sugar-sweetened beverages, fruit juices, pastries, white bread, white pasta, white rice)
Baked goods.