MAC HD FULL!! HELP!!

Hi I really need help clearing up space on my mac... (OSX El Capitan 10.11.6). Im not even lying when i say i have 0KB free space. I cant even open safari. I’ve been searching on this forum for hours but all i see is paragraphs of people using words and terms I dont understand. Im not a tech guy at all. All i was looking for was a simple explanation on how to clean up my mac. I CANNOT download any kind of cleaning app cuz i literally have 0 space. Im losing my mind literally to the point that i feel my mac is lying to me about how much space i really have. I barely have any files on my mac but it says im using up 120gb?? Nah man. I dont believe that Lol. Im on the brink of giving up. Can I move all my files to an external hard drive (WD passport) and then proceed to delete EVERYTHING on my mac?? Sorry if i sound like a douche but im literally losing my mind. Plz help. Thanks

Edit: By “everything” i mean essays & work related documents. I have 0 music (even tho it says i have 2gb worth) and i have 0 movies (even tho it says i have 3gb worth)