go to Settings | System | Storage, and click Show more categories. That will show you what is using the most space. On that same panel you can also run Storage Sense to delete temp files, etc, and you may configure Storage Sense to run automatically in future.

Also, do this:

1. Run Disk Cleanup
2. click the button Clean up system files
3. select your C: drive and select the various check boxes
4. click the More Options tab
5. under System Restore and Shadow Copies click Clean up button and click Delete button to confirm
6. click OK button
7. click Delete Files button.

That should free up a fair bit of space for you.

Note that applications are by default installed to your C: drive. Some applications take up a lot of storage. If there are any applications that you no longer need, you may uninstall them to free up space.