The first thing I would recommend anyone learn how to do is a basic tomato sauce.
This can be the basis of so many meals, with mince to make a bolognese, chilli or shepherds pie, or with lentils / beans to make a cheaper vegetarian option of all of those.
So...
Finely chop a small onion or half a large one
Put a table spoon of oil (preferably olive oil if you can stretch to it) into a heavy based saucepan / casserole dish.
Fry the onion gently for a minute or two until it starts to soften.
If using mince, add the mince now and fry gently for a minute or two until it starts to brown.
Add a tin of chopped tomatoes, a squirt of tomato puree, and a small cup of water. Season with salt & pepper.
Bubble gently on the heat for 30 mins, until the sauce starts to look thick and deep reddish brown. The longer you cook it the better it tastes, but keep stirring occasionally to stop it sticking to the bottom.
I'd try this out with some pasta to begin with - e.g. spaghetti or tagliatelle.
If using lentils or beans (e.g. a tin of large mixed beans), you can add them in with the tomatoes.
For chilli - add a small amount of chilli powder with the salt & pepper, and serve with rice
For shepherd's pie - add some sliced carrots when you add the tomatoes, and a tin of baked beans after 20 mins or so. Once sauce is cooked, top with cooked mashed potato and bake in teh oven for 20 mins.
Extra veg can be added with the onions, e.g. chopped celery, diced peppers, sliced mushrooms.