Cooking can reduce the nutritional content of veg. It can be a particular problem with vitamin C and the B vitamins and minerals like potassium, because they are water-soluble and so leach out into cooking water. That's fine in soups and stews because you consume the water, but it's a problem if you throw the water away. Kale, broccoli, cauliflower, cabbage, courgettes, spinach and peas have a good amount of these vitamins.
Heat also degrades these vitamins, so cook veg that are rich in them gently and in as little water as possible. Steaming, microwaving and stir frying are better options than boiling.
The good news is that many foods we usually eat raw are rich in vitamin C and the B vits (such as watercress, peppers, spinach, tomatoes and lettuce), so a balanced diet will provide you with plenty of them.