Do you have a freezer? My best prep advice often centres round that. I make a massive batch of concentrated tomato sauce with veggies and freeze it in big silicone ice cube trays. Then I defrost them as needed for soup, a pasta sauce, sauce for patatas bravas or to go on top of fish etc.
I always have frozen spinach, green beans and peas in, plus little gyoza dumplings. Then I can always make a quick veg soup, or a ramen-type thing i.e. broth, noodles, green beans and gyoza. Maybe a boiled egg if I'm feeling fancy!
Frozen fish fillets are also very handy, especially if you have an air fryer - they cook pretty quickly.
Store cupboard-wise I would recommend you keep couscous, rice, wholemeal pasta, ramen packets, chopped toms, stock/bouillon, spy sauce, red lentils, maybe red wine/cider vinegar, seeds and nuts.
In the fridge I'd have celery, cherry tomatoes, onions, carrots, feta, halloumi, parmesan, Greek yog.
Why don't you start with a simple meal plan that looks something like this:
.
Monday - Jacket potato with beans or cottage cheese
Tuesday - Homemade soup of any kind (soften onions and garlic with seasonings then add veg and stock/water inc a small chopped potato, blend once cooked and add diary if you want. Or /eave out the potato, add red lentils and don't blend.)
Wednesday - Fish fillet cooked in foil parcel in air fryer with green beans and couscous
Thursday - Eggy bread with some cherry tomatoes roasted in the air fryer
Friday - Wholemeal pasta with some veggies (back to the frozen spinach, green beans and peas!) stirred through with lemon, soft cheese, and other seasonings you like.
This way you're.not relying on ready-meals/loads of UPFs but you're also not throwing yourself into unrealistic levels of cooking from scratch.