Honestly, I think that for a person who hasn't studied any math at the university level, GR is just too difficult a subject. To really know GR, you must know differential geometry, and while I think it would be possible to write a good book on differential geometry that skips the proofs of the theorems that require knowledge of topology, I don't think such a book exists. So you have to know topology just to begin studying differential geometry. This is something that takes months of full-time studies, even for someone who has already studied analysis at the university level.
If you're willing to spend a year or two studying, then you should start with linear algebra and calculus. Then you can try Lee's books on topological manifolds, smooth manifolds and Riemannian manifolds, in that order. But there's a good chance that you will find it too difficult to take the step from calculus to Lee, so you might want to throw in an intermediate step or two. People usually study a book on real analysis before topology.
I think you should forget about GR for now and focus on SR. Start with "Spacetime physics" by Taylor & Wheeler. I haven't read it, but it's supposed to be the easiest intro to SR. So I don't think there's a lot of math in it. The best intro to SR I know is in the first three chapters of "A first course in general relativity" by Schutz. (Yes it's a GR book, but the first chapters are about SR). Unfortunately I think you need to know some linear algebra first.