Q: I AM GETTING A POP UP THAT MY STARTUP DISK IS FULL AND TO REMOVE FILES FOR MORE SPACE . I DO NOT KNOW HOW TO DO THIS

Hello,

First Safe Boot from the HD, (holding Shift key down at bootup), this will clear several cache files & use fewer resources.

Look for some huge files in your Documents folder. For instance, one DVD/Movie may take up 4-9 GB of space, you want at least 50 GB of free space.

Once you clear up a little space...

How much free space is on the HD, where has all the space gone?

OmniDiskSweeper is likely the easiest/best, and is now free...

http://www.omnigroup.com/applications/omnidisksweeper/download/

And if needed, you can use Terminal to run it in admin mode to see files/folders your user can't see...

http://www.macobserver.com/tmo/article/how_to_recover_missing_hard_drive_space/