freeing up disk space

whenever I go and do a disk clean it tells me that I have cleared up x amount of free space on my c drive. which from what I can tell is right. then when I go to do a disk defrag it says that I have 0% of free space on my c drive and that it can't preform the defrag cause I need at least 15%. what do I need to do? it also says that i need to clean up files and delete files that are not used. i have deleted just about everything i can without crashing the computer and just starting over. HELP!! thanks so much to anyone out there that can point me in the right direction.