DiffGeo combines calculus and geometry. While it invades geometry with the methods of calculus, it also advances calculus onto richer geometric frameworks. For background, the equivalent understanding of a calculus and differential equations survey is desirable (I see you have that!). Also, since it gets into research level areas, a familiarity with the grammar and basic content of abstract algebra and general topology is desirable. Much of the online instructional material is physics-oriented. That covers much of the historical (nineteenth century to early twentieth century) interest of the subject, but not so much the later pure mathematical interest, found mostly in books and journals.
Here are a few links:
the position of DiffGeo within the mathematics world ->
http://www.math.niu.edu/~rusin/known-math/index/tour_geo.html
Math Atlas: geometric areas of mathematics
a short introduction to DiffGeo terms ->
http://www.wikipedia.org/wiki/Differential_geometry
Wikipedia: Differential Geometry
two physics-oriented online sets of lecture notes ->
http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/tc.html
Introduction to Differential Geometry and General Relativity
http://people.uncw.edu/lugo/COURSES/DiffGeom/dg1.htm
Differential Geometry and Physics
(don't forget to enjoy what you are doing)