Can I get a quick rundown on nutrition science?

>QUICK

Whats the starting salary?

>RUNDOWN

Exercise, eat the calories needed to maintain weight, get all your %DV vitamins from natural sources, get the recommended minimums for protein/fats/carbs, and get enough water and fiber to keep the pipes clean. Do those and you'll be healthy (nutritionally).

Your body needs food basically for fuel and making body tissue. There are micro and macro nutrients.

MACRO

You dont need any carbs, but they are a cheap and easy food source. There are complex carbs (starches) and simple carbs like sugars. Although low carb is a big trend right now, there is no reason to avoid them.

You need fats for production of some hormons and shit. Its better to consume unsaturated fats. There is something called omega 3 fatty acids and they are good for you. You find them in fish like salmon and also in linseed. Too many bad fats ( eg saturated fats from fatty meat like pork) are terrible for your health.
You body needs protein, because he will disassembly them back into amino acids, whcih are the building blocks of your own proteins, which are the building blocks of almost everything you are. Meat and eggs are the best protein source, because they have similar amino acid composition like human tissue.
If you eat plant protein, you will always get excess amounts of some amino acids and lack some others. mixing plant food makes this situation better.
Unless you plan to be a body builder you dont have to worry about right amounts of protein too much though.

If you eat too much your body will store it as body fat, some bodyfat is okay, too much is bad.
If you want more muscles work out and eat a bit more then you need for just maitaining your weight.

Fiber is "wood like" plant matter that you generally cannot digest, or can only digest in small quantities. However its good for your digestion to eat plenty of it and it makes you poop better.

MICRO

There are different kinds of so called micronutrients. They are certain substances that your body needs to survive, but only in small qunatities. The most important of these are named the so called "vitamins". make sure you get enough of those. They are complicated. There are many of them. Just eat enough of a variety of vegetables and fruit (mix it up a bit) and you will be fine.

Where should I get fibre? I eat cereal so that should do it

It should be whole grain cereals, not your kids stuff. Fruits and veggies also have fiber.

your main source of fibre should be green vegetables like brokkoli and cabbage and shit.

Whole grains got some as well, but they also have kinda high calories, so not good if you want to lose weight. Try oats if grains.

"cereal" usually means sugar bombs.

Trying to gain weight desu

The problem with this shit you mention is that I can't just have a broccoli or cabbage for breakfast while you can poor a cereal

Maybe you can stir fry it but it's not easy and requires other ingredients too

The healthier cereals are fine if you're lazy. Just check for fiber and low sugar.

Most, if not all, cruciferous vegetables are loaded with fiber water, and scant amounts of other certain vitamins (Like D). All cruciferous vegetables stimulate enzymes in the liver and there is a fair amount of research that suggests they may have protective qualities over cancers *1, *2. These vegetables fight for bioavailability within the liver (Hepatic clearance, check it out!), so they can interfere in very large quantities with some medications.

1 tandfonline.com/doi/abs/10.1080/01635581.2001.9680607
2 ncbi.nlm.nih.gov/pubmed/16284385