In order, I think you should :
1-Make sure you are good with basic vector and matrix math
2-Make sure you are good with basic newtonian physics (mechanics, gravity, etc)
3-(optional but very helpful) become familiar with coordinate changes (from cartesian to spherical and others like cylindrical)
4- Learn special relativity before general relativity... I specify it because sometimes it is not obvious that special relativity is a prerequisite for the general relativity because the names may be misleading (http://en.wikipedia.org/wiki/Special_relativity)
5- Learn about tensor (http://en.wikipedia.org/wiki/Tensor)
Now you can start reading about general relativity