Joseph has an old Mac and he has tons of videos, music, and other stuff on it. He's deleted a lot of it. But he still has over 172GB of files he can't find. How can he get rid of them? Leo says to download Disc Inventory X. It will show him what and where those files are and will help him get rid of them.
Generally, though, Leo advises just starting from scratch once a year. He should back up his data and then format the hard drive and reinstall the OS. That way it's like starting over and will be just like the day he got it. For Mac, he'll just have to reboot the machine and hold down the CMD key + R. It will allow him to then rebuild and install the OS. Then he can just update it and he'll be all cleaned up and ready to move forward.