Burn more kalories than you eat. It's that easy! But of course, eat enough!
You should also avoid industrial sugar and white flour. Eat poultry, fish, vegetables, fruits instead. Drink tea and water instead of softdrinks. In case you ask for a specific diet, a standard low carb diet works pretty well but you should start working out. 'Working out' doesn't mean that you need to go to a gym. You can train at home, there's even no need to buy equipment (search for 'bodyweight training' on YouTube).