Follow recipes exactly as they say. If you don't have an exact ingredient or piece of equipment, look for a different recipe.

When scooping up flour, don't "pack" it by tapping the side of the cup. You want air to remain and for it to be fluffy. If you pack it by trying to shake it so more flour goes into the cup, you'll be adding too much flour.

Low heat and time are your friend. Cooking something on high heat will burn the outside while leaving the inside cold. Some recipes will want you to do that. But when it says "low heat," it means you have to cook it on low heat so the food is heated all the way through. If a recipe says to bake something for 20 minutes at 200°, you can't bake it for 10 minutes at 400°.

Start with simple dishes with maybe four or five ingredients tops. Stuff like pasta or soup is a good place to start. Maybe learn to grill veggies one type at a time.

As far as nutrients goes, microwaving will maintain the most nutrients of the food because it's not actually burning anything away. It won't always taste good, however. Boiling will take out the most nutrients because they get soaked up by the water, which is often tossed out or turned into broth that isn't eaten. Baking, grilling, broiling, etc. are on a spectrum between these two. The more the food is burned/charred/fried/whatever, the more nutrients are cooked off. If you're counting calories, keep this in mind, especially if you're trying to bulk up or gain weight.