If you are looking to clean up Windows 10 after an upgrade or clean install, or just as a good general maintenance priority, there are some things you can do to streamline operations and remove unwanted junk. Most of it is pretty standard, similar in nature to previous versions of Windows but there are a few differences and some new features that may make finding the clutter a bit more difficult.
The following steps will help guide you through the process of getting Windows 10 clutter-free in preparation of making a backup image or just for general principles.
The first thing you probably want to do if you've performed a Windows 10 upgrade or clean install is open windows file explorer, click on the C:\ drive, find the windows.old folder and delete it. This folder contains your previous OS installation and is only there for the purpose of reverting to your previous installation. In my experience, the process is flawed and fails to work correctly on most systems, or further complicates the probably already borked configuration to the point where you'll probably have to reinstall anyhow. I delete it. If it won't delete, don't worry, once you run the system files disk cleanup option, making sure to select the "old windows installations" option in addition to everything else listed, that should automatically be cleared out.
If you have done an upgrade, I recommend performing a clean install in ALL cases anyhow, so this point SHOULD be moot, but I know some users are not inclined to do a clean install as they have irreplaceable software installed. Still, it's always a good idea to do a clean install anytime you move to a new operating system or as a periodic maintenance procedure, so if you wish to do that the following link will explain how:
How to ditch the Windows 10 upgrade and perform a clean install.
Or, if you just want to do it right from the start, and haven't yet done so, this might be more helpful.
How to do a clean install of Windows 10 without upgrading.
If you already have done a clean install there will be no windows.old folder, so you can skip that step.
Next, right click your C:\ drive in Windows explorer and select properties, then click the "Disk cleanup" button, or open disk cleanup from Administrative tools.
In the disk cleanup window, check every listed item checkbox (Except for the "Downloads" folder if it is listed, that you will want to clean out manually so you don't accidentally get rid of anything important. Don't confuse the "Downloaded program files" and the "Downloads" folders, they are not the same. It is ok to select the Downloaded program files folder. If there are ANY folders listed that you KNOW contain important PERSONAL files, move those files to a more permanent location before proceeding.) and then click ok. Any files that are not important TO YOU, are not going to be missed here. Windows isn't going to delete any critical system files during this process.
A verification box will open asking "Are you sure you want to permanently delete files"? Click delete files and wait for the process to complete.
Then go back into disk cleanup and click on the box that says "Cleanup system files". Repeat the cleanup process.
Now go back into Windows explorer and click on your C:\ drive in the left hand pane. In the right hand pane, open the Windows folder. Find and open the "Temp" folder and select everything. Delete the contents. If there is also a Temp folder on the root of your C:\ drive, clean out that folder as well. Everything contained in the Temp folders is safe to delete, hence the reason they are in the temp folder.
The next step is optional, but can clear out a lot of space, especially if you have been using the default configuration. Right click on "This PC" in the left hand pane of Windows explorer, or open the "System" applet in control panel. Click on the Advanced system settings option.
Click on the System protection tab.
Click on your C:\ drive and then click on "Configure". In the configuration settings you have a couple of choices.
One, if you have other backup software, such as Acronis True Image, which you use for the creation of system backup images to restore from in the event of an OS corruption or drive failure, then you can safely turn off system restore.
You should also click the box that says Delete next to "Delete all restore points for this drive". If you do not have other backup imaging software, get some, but until then, just click on "Delete all restore points for this drive" and then follow up by setting your drive space allocation for system restore to no more than 10GB. 5GB is probably plenty.
Next, if you have chosen to keep system restore enabled because you do not have other system backup software, go back to the Protection tab and click on "Create" to create a new restore point. When finished, close everything out.
Restart your computer and go back into Windows explorer. Click on your C:\ drive and select properties. Click on the Tools tab and then click Optimize.
Click on your C:\ drive and click Optimize. Perform optimization on any other connected drives once your C:\ drive is complete. Another thing you can do here is to click on the "Change settings" button and then DISABLE the scheduled optimization by removing the check mark next to "Run on a schedule".
Automatic optimization can be scheduled to run at specific times, but if it begins running while you are doing something else it can use significant resources. I recommend simply running optimization about once a week, manually, if you can remember to do so. If not, then just leave it enabled BUT maybe set the schedule to a less frequent designation like weekly or monthly.
You system should now be pretty well cleared out of the majority of cruft and accumulated garbage, and the file system should be fairly well optimized on disk. There are a number of other procedures that can be performed BEFORE running disk optimization, such as removing unwanted or old programs from the Programs and features applet in control panel, but since I have no way of knowing what applications you may or may not have on your system, or whether you've already done a clean install and have no applications currently installed, I'll leave that part of the process up to you.