System using 383GB of my hard drive??

I was downloading a movie to my hard drive and I got a message that it could do it, but it was close to being full. At that time I had only about 7GB remaining to use on my 1TB hard drive. I deleted many duplicate/unneeded photos, videos, applications and documents and now have over 175GB available, but I checked and the System itself is using 383GB of storage, over 1/3 of my hard drive. That seems very excessive to me. I do not use Time Machine, although I did previously with an external hard drive. I am a newer Mac user and while I would know where to go on a Windows machine to delete system files that I know the computer can function without, I cannot seem to access those on the Mac. How do I get in to find out why the system is so large?