If you're not overweight and eat roughly the same number of calories as you burn up daily then you shouldn't become overweight.
If you're already overweight the first thing to acknowledge is that there's no quick fix. To lose some weight you need to burn more calories than you're consuming. For most men the solution is to:
always eat breakfast
eat meals that contain more starchy foods (such as pasta, rice, bread, potatoes) and less sugar and fat
drink less alcohol
snack on fruit rather than chocolate or crisps, for example
reduce daily calorie intake by about 500 calories
exercise for at least 30 minutes on a minimum of five days in the week (such as brisk walking, swimming, cycling, running)

Should I see a doctor?
If despite these measures the weight isn't falling off, then some advice from the doctor, practice nurse or dietician may help.