Hello, this is Emily.
You shouldn't have to buy software to clean up your disk.
You can type Disk Cleanup in the search on the taskbar and press enter to launch the app. When launched, click on the "clean up system files" button, it will give you the options to delete files that can be removed safely within this app.
In addition, your system drive (C drive) is honestly not that big. Windows has to be operated on this drive, but you can consider saving your personal files to the D drive. Your personal files are those in Desktop, Documents, Pictures, Downloads, Music and Video folders for example. They are not system files, so they can be safely moved to the D drive to make room in C drive.