I'm no expert, but here are some resources I've found useful. Benjamin Crowell's online textbook
http://www.lightandmatter.com/ has a nice chapter introducing relativity, At the same site, he has a book on general relativity--although I haven't got very far with that one yet. I'll add my voice to all those recommending Taylor & Wheeler:
Spacetime Physics! It uses algebra, trigonometry and a little calculus in places.
Tensor analysis, which Altabeh mentioned, is a branch of linear algebra.
MIT have courses of video lectures on linear algebra, single and multivariable calculus, differential equations.
This series on multivariabel calculus from Berkeley is good, especially his treatment in the final few lectures of Stokes' Theorem. The
Khan Academy is justly famous for its videos on mathematical (and other) subjects. They're mostly about 10 minutes long, and offer a gentle way to get started on a new subject. After struggling with several online explanations of tensors, I found these books by Ray M. Bowen and C. C. Wang refreshingly straighforward:
Introduction to Vectors and Tensors, Vol 1: Linear and Multilinear Algebra
Introduction to Vectors and Tensors, Vol 2: Vector and Tensor Analysi
To understand the terminology used by mathematical texts, it helps to read at least a little bit about abstract algebra: sets, functions, groups... I keep returning to Sean Carroll's
Lecture Notes on General Relativity and using it as a guide to what math to learn next. I've also dipped into Blandford & Thorne: http://www.pma.caltech.edu/Courses/ph136/yr2008/, which treats Newtonian physics and special relativity in parallel. In the final chapters it turns to general relativity.
Gerard 't Hooft has made this syllabus of suggestions for anyone attempting to learn physics from the internet:
Theoretical Physics as a Challenge.