There are a number of aspects to doing "maintenance" on Windows, or general housekeeping if you prefer to call it that. Generally speaking we start down this path when our machine is running slowly. Clearly there can be many causes of this, however there are some common solutions to this that also help with disk space issues and malware hunting.
One of the most important things to bear in mind is to consider the source of the advice you are following! It is all too easy to search on the internet for a solution, find something and only make matters worse! Firstly, check the source of the advice! If it is a support/help article by Microsoft like some of those referenced on this page, then you are in good hands but posts on Microsoft's forums might not be as good. Secondly if you are downloading and installing a utility, then be very careful. Can you trust the supplier? Is it coming from a suspicious download site? In both these cases the following sites can help your assessment:
Whilst a bit of a pain, I could recommend more than one check.There are two hardware changes that can speed up your computer. Adding more memory or RAM will help when you need lots of software open and running at the same time. Moving to an SSD will help with start-up times of both your system and each application.
The best option is to format the hard drive and start again but this can be difficult and painful and sometimes very hard to get back to where you were with everything setup and working, albeit slowly. Windows 10 does have some handy options to help with this and make things much easier, take a look at Recovery options in Windows 10 and the "Reset your PC" options.
My go to utility for helping understand what is taking up all the space on my disk is WinDirStat - Windows Directory Statistics, which is a great visual tool. Of course, once you find where all the space is being taken up, the next problem is how to correctly free the space up.
The important point to note is that if you find a folder under C:\Windows (or wherever you have Windows installed) then you must proceed with extra caution. As an example C:\Windows\WinSxS often takes a lot of space, so visit Tips to free up drive space on your PC - Windows Help for some official advice on what to do. Another good tool is "Disk Cleanup", see Disk cleanup in Windows 10 for that, which does also exist on Windows 7 and Windows 8, then there is Free up drive space in Windows 10 - Windows Help which applies to Windows 10. Hopefully though you are getting the general idea.
One way to increase the free space you have is to add another hard drive. The best option for performance is to add an SSD and copy everything onto it, such that it contains the operating system and is your boot disk. Then you can have a second, larger magnetic hard drive for storage, in fact some systems come configured like this. The next challenge is moving stuff over. With some things, like Documents, Music, Pictures and Videos it is easy to add a folder to the library and then move everything into that. You can move your entire user profile directory but it might be better to leave some of this on your SSD as certain apps install into the profile and others use it for temporary files. The best way to relocate stuff is to use an NTFS Junction, see Useful Commands for some help with this.
This can be a tricky thing to deal with, however I generally use Autoruns for Windows - Windows Sysinternals | Microsoft Docs to see what is getting started when the system boots. Once you know what needs removing then you are trying removing the software properly before taking more drastic measures, however Autoruns does allow you to disable stuff.