Disk Maintenance

I AM A NOVICE, SO PLEASE FORGIVE ME…..
wHEN I TRIED TO DEFRAG IT WOULDN’T CONTINUE BECAUSE OF A FOLDER PROBLEM. THEN WHEN I STARTED SCAN DISK, IT WOULDN’T CONTINUE BECAUSE THE TOP LEVEL FOLDER IN DRIVE “C” WAS FULL. wHAT IS THE TOP LEVEL FOLDERAND HOW CAN I DELETE FILES FROM IT? THANKS FOR ANY HELP AND THANKS FOR ALL THE GREAT TIPS!!!