You need to understand what is taking up all of startup disk space on your Mac. Then clean up your startup disk space based on the following ten aspects.
1. Clean up the cache file on Mac
2. Delete unnecessary language packs
3. Delete the old iTunes library backup
4. Remove old iOS backups
5. Remove iOS software updates
6. Remove unnecessary applications
7. Clean up your photo library
8. Clean up your downloads, movies, and music folders
9. Clean up your desktop
10. Empty out the trash can
If you find it difficult to clean these ones manually, you can download some Mac cleanup tools (such as my recently used iMyMac's Mac Cleaner software) to help you clean up these things.
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