Taking notes is something that anyone who works with intellectual matters has to do. You typically start in high school, when the things you learn are no longer trivial and can no longer be retained by heart just by hearing them, and it goes on throughout a lifetime for many of us. A lot of what makes a good engineer or scientist isn't what they can remember, but how good they are at finding the answer, whether that's by referring to your notes from previous problems you've solved, searching for the answer through experiments, or just Googling for it.
If you want to retain that knowledge and not waste time when you need to do something that's close enough to something you've touched in the past, having a well organizing notes taking system is crucial. Over the years I've used a lot of tools myself, and I will review what I consider 3 of the best and most popular options here.
Apple Notes
If you use an iPhone, like many of us do, the default notes app that comes with iOS is Apple Notes. When it first started, this was barely more than a basic notepad. Now however, the company has improved the software to offer more advanced capabilities, like adding images, drawings, locations and so on. Still, Apple Notes can only be considered as a basic solution for notes taking. It's fine for a student or if you only need to take casual notes, but few people would use it in a professional environment.
Pros: Very simple to use, comes pre-installed on all iOS devices, syncs between your devices, free.
Cons: Doesn't offer a lot of advanced functions, doesn't have Windows or Android clients.
Microsoft OneNote
OneNote comes by default with Microsoft Office, which is why a lot of people adopted the software. In most companies that I've dealt with, OneNote is the default option. It's a very powerful option that Microsoft offers for free, and it supports most data types. It also provides the ability to lock certain sections behind a password for more sensitive data. If you're in IT or just a regular user of Office, then this may be the best option for you.
Pros: Integrates with Microsoft Office, has clients for every platform, free.
Cons: Requires a Microsoft account, some may not like the layout of notes.
Confluence
As the third option, I listed Confluence but what I will cover is basically any type of web based online notes taking system. Confluence just happens to be the most popular option in business, at least from my experience, but you could use a wiki or any number of options. The benefit of such a system is that it's always available in an easy to read web format, and can be shared easily, since everything is on the web. The downside of course is that it requires you to always be connected in order to access those notes. This is a solution that many businesses put in place, but anyone can sign up as well, if you prefer a web based option rather than a client software.
Pros: Good presentation, easy to share notes.
Cons: Not free, requires a constant connection.
Conclusion
The option you pick is up to you. Each of these options have their own strengths and weaknesses, and there are millions of people using each of them. I think it's more important to actually pick something and stick with it, because you never know when something you learned years ago will be relevant to you once more. Personally, after using a myriad of options and not getting on board fully with them, I ended up making my own data management system, but that's probably not for everyone.