I'm doing a PhD in food technology, so I have a bit of insight into nutrition from a scientific point of view (although I'm not a nutritionist by any means). In general you can say that it's good to avoid excess sugar, so the amount of candy, cake, soda etc you consume should be as low as possible. Fast food that doesn't fill you up while having a huge energy density is also something that's good to avoid. After that it gets more complex.
What most people in the west are hugely lacking in is dietary fiber, the intake of which is really clearly associated with a reduced risk of diabetes, colorectal cancer and obesity. Fiber is good for gut function, it reduces the energy density of food (i.e. fills you up with less calories), and is food for the good bacteria in your gut. The final point is something that's received more and more attention lately, as there's now a lot of evidence that gut microbiota has a huge effect on health.
Of course, saying you should eat more fiber is a bit simplified. The type and source are also important, and it's also most likely important to get a good mix of different fibers. So a mix of wholegrain, nuts, vegetables, legumes will give you a nice mix of fiber and other nutrients. The thing with fiber is that at least part of the health benefit can come from components connected with certain fibers, like phenolic compounds, so a pure fiber might not give the same benefits as e.g. cereal bran or whole nuts.
When it comes to wholegrain cereals, the processing also has an effect on how good they are for you. If you have a very finely ground flour where the structure of the grain is totally broken down, it will give a faster blood sugar response than a more coarsely ground. Therefore when it comes to cereals, something like oat porridge made of wholegrain flakes is better than a puffed wholegrain. On the other hand, if you absolutely want to eat puffed cereals, a wholegrain one is most likely better than one made of white flour. This is also part of the reason that you can't just talk about carbs in general being good or bad, it all depends on the type and the matrix. Personally, based on the scientific literature available I'd say that eating good quality carbs as part of a varied diet is good for you. What many people forget is that cereals aren't 100% starch either, e.g. oats can contain up to 20 % protein, up to almost 10 % fat, and 10-ish % fiber - so about half of the weight of wholegrain oats is actually carbs.
When it comes to protein, eating a relatively high amount can help with weight loss because its satiety response. However, overdoing it causes stress on your kidneys so in the long term eating excess protein should be avoided
When it comes to eating or not eating animal fats, that is a bit tricky. There's epidemiological studies that indicate that vegetarian diets are healthier than meat containing diets. However, with these types of statistical studies it can be difficult to control all relevant factors and they don't reveal any mechanisms. Of course, from a environmental POV it's better to eat less meat than most people in western countries do (and eat better quality, not factory grown meat). Fish is pretty indisputably good for both health and the environment unless you eat some overfished species.