When I was 17, I spent a week in Cornwall with 4 friends. The mother of one of them had given him a leg of lamb from the freezer, so we could cook a roast on the Sunday. Then we discovered that none of us had a clue what to do with it!
I had a vague idea that my mum used to parboil potatoes before roasting (although I didn't know that was what it was called). Between us, we figured that a leg of lamb would take a lot longer to cook than potatoes, on account of being bigger and not part-cooked. 
We somehow muddled through and had a passable meal, despite having horribly overcooked cabbage.
I left home 18 months later and bought myself a Good Housekeeping cookery book. It taught me almost everything I know about cooking. I still have it, more than 40 years later and it's still my go-to book for any basic recipe, like meringue or choux pastry.
I think it's easier to learn to cook now than ever before: the internet and the huge amount of cookery programmes on tv mean that you can actually watch how to do things, over and over again if need be.
I have my favourite cooks whose recipes seem 100% foolproof (Nigel Slater, Mary Berry and Delia). And I tweak seasonings, and have developed a few tricks of my own, eg a teaspoon of black treacle and a shake of balsamic vinegar gives a lovely piquancy and richness to a gravy.
Having said all that, I think it's hard to learn to cook if you don't love food. All the poor-middling cooks I know are people who simply aren't very interested in food. DP and I both love to eat, and are both good cooks, but his mother and 2 of his siblings somehow manage to turn out meals of astonishing blandness.