Windows 10 can at the bare minimum, take 20GB of disk space, and commonly takes up over 30GB when it's in daily use.

In my tests I managed to get Windows 10 to take up 8GB of disk space instead of the 22GB it was taking up previously.

This method doesn't involve installing any extra programs - the functions are all built into Windows 10.

1. Go to the start menu and click it so it's open
2. Type 'Command Prompt' - as you begin typing it should show up.
3. Right-Click the program 'Command Prompt' and choose 'Run as Administrator'
4. In the new window that appears, type

Now press Enter. This will tell you if the system is already in the mode where it's trying to compress the files to save disk space.
5. If it's not compressed already, you can type:

Now press Enter again. This may take a while. In future, the disk space used by Windows 10 will be kept small because the system will be able to use the original files in their compressed state. You won't notice any changes to the way your documents look or open.
6. To reduce the amount of disk space Windows 10 uses even more, you can reduce the size of the hibernation file (the file Windows 10 saves everything to if you put the computer to sleep). With Command Prompt still open, type:

7. If you want to remove the hibernation file completely, you can use:

8. If you want to restore the default hibernation settings use this command:

9. You may be prompted to restart.

The disk space used should be drastically less, leaving you more space for your own files.

Also, with the Command Prompt open (as an Administrator), run:

This should throw out any temp files.

Additionally, You can manually empty the temp folder by opening Command Prompt from the start menu, and typing:

Then press return or Enter. This will open the temp folder, delete whatever you want in here (make sure no programs are open to be safe).

You can also save space by clearing out log files that build up over time.
Run Command Prompt (as an administrator) - like in step 1 to 3.

Run these commands to clear out log files:

Oh, I almost forgot:
DISM++
(https://www.chuyu.me/en/index.html)
Alternative mirror here
You can reduce the footprint of Windows 10 even further.
I've been testing, and with all updates applied, drive compression turned on and DISM++ clean performed I can get the footprint of Windows 10 down to 7.8GB (which still sounds a lot to me, however it's better than the 22GB it did take).