Barring a medical cause, when you gain weight, the usual culprits are too much food and poor choices. For example, large portions of fatty meats, more than the occasional candy bar, frequent fried indulgences along with the heavy use of sodas are a sure recipe for gaining weight.
If you salt your food heavily, it's most likely it's just a habit your palate can be weaned from. Salt causes you to retain water and is unhealthy for your heart. Try a salt substitute, or look up some of the many herbal saltless mixtures.
Before you start your diet, pick up a good nutrition reference guide. Such books give you a complete picture of calories, carbohydrates, fats, fiber and protein content of foods, along with figures on vitamin and mineral content.