What I was told that I found to be helpful was to try to think of offering them a varied diet, lots of healthy fruits and veg, protein sources (which can be meats, fish, seafood, pulses like lentils or chickpeas, nuts, eggs, etc.), starchy foods like rice, pasta, potatoes, and dairy (yogurt, cottage cheese, milk, hard and soft cheese - toddlers should still have about 500ml a day of milk or an equivalent in dairy after they stop formula, so if he likes milk that's a good way to get in some healthy dairy) and try to think about having a good balance between all of it over a week rather than a day. You will have days where your LO eats toast and peanut butter and apples, and then the next day they eat lots of fish, rice and carrots. If you look at it as one day, it doesn't look totally nutritionally complete (though not bad either), but if you look at all the foods he might eat over a week, it ends up looking much more varied and nutritionally rich. So I'd move away from trying to pack everything into a nutritionally complete day and focus on offering lots of whole foods that you as much as possible cook yourself and offer him as much as he wants. He'll eat til he's had enough. As long as it's all relatively healthy, you don't have to worry about portion sizes. If he was eating, say, chocolate, you'd probably want to limit that, but I wouldn't put a limit on carrots or rice or sausages (assuming maybe you aren't offering sausages at every meal). He'll know what he needs and he'll eat what he wants. You can offer more if he's finished.
As for learning to cook more and from scratch, I'd really recommend finding a few really good cookbooks to buy that have lots of nice recipes that you like and with healthy ingredients (and ingredients that are normal enough to find in your local shop, not some weird stuff you have to go to a specialty shop for). I used the River Cottage Baby and Toddler Cookbook a lot when my daughter was younger. I've also heard people say that Jamie Oliver's 15 Minute Meals is good too. Also, I really like using Pinterest for finding recipes. I don't really get it for much else, but there are loads of recipes on there, many of them geared towards family meals. Even if you just find 7 that you like and are relatively easy to make, you can plan a week's worth of dinners that way. Then just keep trying them over and over again until you get comfortable with them and you're happy with how they turn out. Then you at least have a week of healthy meals you feel confident cooking. You might also look to get a slow cooker (and maybe a slow cooker cookbook or look for recipes online). I find it helps me a lot to cook something decent on days that are really busy without much effort. It's hard to screw up something in a slow cooker, even if you don't know how to cook.
