Write down what you plan to eat for that week. Then take each meal at a time, check what ingredients you need against what you already have and then add those that you don't have to your shopping list.
Example:
Monday Breakfast: Cereal/toast, fruit juice, tea/coffee etc
Monday Lunch: Mixed veg soup, toast or bread rolls
Monday Supper: Shepherds Pie, peas and sweetcorn
Tuesday Breakfast: Boiled egg and soldiers, drinks etc
Tuesday Lunch: Cheese on toast
Tuesday Supper: Spaghetti Carbonara
etc etc
List:
Cereals
Bread - sliced and part cooked baguettes (for example)
Fruit Juices
Milk
Broccoli
Onion
Garlic
Leeks
Carrots
Cauliflower
Chicken/Veg stock cubes
Minced lamb
Potatoes
Mushrooms
Frozen peas and sweetcorn
Eggs
Cheddar
Spaghetti
Parmesan or other hard cheese
etc etc
You will have lots of store cupboard/freezer stuff, so adapt shopping list accordingly.
I usually start with Sunday and a roast supper, then make something with leftovers like a risotto, curry or pasta dish for the Monday evening.