Start with food that look or are similar to what they started life as. A piece of fish is just fish - nothing done to it or added to it, whereas battered fish or a shop-bought fishcake may have lots of added ingredients - not all of them healthy. Same with veg, eggs, pulses or cuts of meat. Cook them from their natural state, rather than, for example, buying smoked bacon that has probably just had artificial smoke flavour added, or pre-chopped veg that has been dipped in dilute bleach to stop it discolouring.
If you base most of your food around these, it's fine to have some 'processed' foods that are fairly simple like wholemeal bread or natural yoghurt, brined olives or good cheese.
So a healthy mainly unprocessed day's food might be:
Home made porridge with blueberries and chopped bananas
Or natural yoghurt with berries and nuts and a drizzle of honey with cinnamon
Or a home made smoothie with fruit, natural yoghurt or milk and some nuts or seeds.
Lunch might be poached eggs on wilted spinach with grilled tomatoes
Or avocado on wholemeal toast with salad
Or stir fry veg and rice
Or home made veggie and lentil soup
Dinner might be steak with home made garlic sweet potato wedges and steamed broccoli
Or roast hake or salmon on a tray of chopped veg (eg onions, fennel, asparagus tossed lightly in oil with lemon and herbs
Or tray bake chicken with baby potatoes and mediterranean veg
Or a lentil or bean based casserole with a tomatoey sauce and some brown rice.
It can be expensive, but if you only eat this good home made food, and don't buy takeouts, snacks, baked goods as treats, too much booze etc, it isn't too bad. On a budget eggs, pulses, chicken and beans are good.