How should I fix my mac?

I turned on file vault a long time ago, but now I don’t need it. I have 1 GB of memory left and I think that all the other memory (about 60 GB) is being used up . I tried turning file vault off but it says I cant do that and I need more memory.

Do any of you know how I can fix it, I tried deleting some unneeded stuff but it hardly made a difference.