I used to be like you before i had dc. My mil is a good cook but would never teach dh. She did however buy all these fancy cook books - we didnt understand them, they had words i didnt have a clue about in, tons of ingredients etc.
I think you need to tackle it one meal at a time, so instead of trying to change breakfast, lunch and dinners for a whole week in one go, just add one new healthy dinner. Then when youre confident making that new healthy dinner add another into the week. Then once youve got a good selection of dinners, look at breakfasts.
Tbh i think a cheese sandwhich is ok for lunch, if youre getting a proper evening meal.
Now we can cook. Someone told me that cow&gate (the baby food company), actually did a good weaning guide, using homemade recipes, so i ordered it. Then as lo got older they sent these little magazines with recipes for thinks like chilli, spag bol, fajitas, cottage pie, etc. These recipes were soo simple to follow with simple ingredients that i learnt to cook! I think they have a lot of these recipes on their website too.
Also i use a lot of frozen ingredients - for example i buy peppers, onions, mushrooms ready chopped and frozen (a large handful is about 1onion/1pepper), also things like peas, sweetcorn, baby carrots, mince beef, etc are bought frozen. It makes it easier as you dont have to plan in advance for a lot of meals then.