If you have used a mac for awhile, and if you are virtual hoarder like I am, you will find that even though you have deleted files you still have a very bloated hard drive. There is a reason for this. Macs let you recover deleted files in case you change your mind. In order to do that it has to continue to store them, even though they may not be visible. This adds up over time.
To really truly irrevocably delete the files, go to applications, select utilities, then disk utility. Your drives will be listed on the left (even flash drives if you want)
Click the drive you want to clean up then click erase.
Don’t freak out – this will not erase everything.
Click erase free space.
Choose zero out deleted files. You can choose the others but if you have never done it before it will take a long, long, loooonnnggg time….