See what uses the space
On recent macOS versions open System Settings > General > Storage. Review applications, documents, backups and large files before deleting anything.
Remove items you can verify
Check Downloads, unused apps, duplicates and old iPhone or iPad backups. Move important media to a verified external drive and empty the Trash only after confirming the copy.
Avoid random system cleanup
Do not manually delete system folders because they look large. If the Mac freezes, will not start or contains important data, stop before reinstalling and request help.