You are the only one who knows what interests you, so in your Nature Notebook you can keep whatever information and thoughts you want.
However, it's a good bet that once you really get into Nature, one thing will lead to another, and before long you're paying attention to all kinds of things, from trees to earthworms, algae to fish.
If that happens, you may want to organize your notes, maybe according to the classification system outlined on our "Pigeon-Holing Our Discoveries" page.
Plants would be in one part of the notebook, animals in another. Within the animal section, birds would be in one place, insects in another. Within the bird section, you might begin with the loon family, and continue with the grebe family. This system has the advantage of placing like kinds of information together, instead of having BEANS next to BEAVERS.
On the Internet, the free-to-use iNaturalist.org does all the organization for you. If you have a decent picture of something you can't identify, and want to know what it is, someone in the worldwide community of iNaturalist users will help you.
The iNaturalist.org website is so easy to use that all you have to do is go to the website, read its simple, step-by-step instructions, and jump into it. To see the entry page, click here on iNaturalist.org.
Once you've sent your image to iNaturalist, and the thing you've photographed is named, people all over the world will be able to see your picture of it. Not only that, but when you tell iNaturalist where and when your photo was taken, the distribution map of that species will show a dot at that location, and anyone in the world will be able to click on that dot and see who was there taking the picture, exactly where it was, and when, plus they can read any notes about it you want to share.
iNaturalist is such a helpful website that researchers worldwide use it. For example, if someone on the opposite side of the world from you is studying oak trees, and they see that oaks occur in your area, your photo and information will be there for them.