Knife skills. Keep them sharp. Mise en place. The importance of salt and pepper. Searing techniques for cooking meat and how not to do it. Roasting vegetables 101. The basics of finishing pastas and reducing sauces. The fact that you can make your own dressing in no time.
You can probably cover all of that quickly and in enough time to ask the right questions moving forward and begin your jorney - IF you had a curriculum. If it's self study it takes longer to figure out what you don't know :) For me it all started with "why does my Grilled Cheese suck? This should be easy" and it's been all downhill.
The Food Lab by Kenji Lopez-Alt was a big window into this for me because of the approach around time and temperature, and the book takes you through all of this stuff by section.
Cooking for over 15 years at home and still only learned this recently despite already knowing about it. I had to grind out a chip from one of our knives, and I realised when I was done it was much sharper than I usually get them. I hadn't been doing it quite right all this time.
> Mise en place
Asian wok-based cooking is great for learning this. You don't have time to mess around. It's something I've been trying lately (my frying pan stir-fry was always pathetic so I never made it at home, but now that I have a wok I can do a decent one).
+ 2 parts olive oil
+ 1 part honey (try and make it good wildflower or clover honey)
+ 1 part squeezed lemon juice (add some zest as well if you're feeling energetic—I hate cleaning graters)
+ Salt & pepper to taste
Here's my default dressing: salt & pepper to taste. Bit of lemon juice. Bit of dijon mustard. Balsamic vinegar and EV olive oil. Adjust relative quantities to taste and based on what you have in your salad. Gets rave reviews and could not be simpler. ;-)