There are several ways to clean up your Mac:
1. Sort all files by size and delete old large files.
2. Use third-party software for cleanup. For example, I recently used the iMyMac Mac Cleaner software.
3. Empty your trash can
4. Uninstall the application you are not using
5. Clean up huge iTunes backups for iPhone or iPad
6. Clear temporary files
7. Find and delete duplicate files
8. Delete language files
9. Clean up large attachments in Mac Mail
10. Clean up the download folder
11. Use the storage tools in macOS High Sierra