Preparation for GIT (Geometric Invariant Theory) I am trying to read Mumford's Geometric Invariant Theory, however, I find my knowledge in algebraic geometry is inadequate. My knowledge is at the level of  Hartshorne's Algebraic Geometry. Mumford cites a lot of results from EGA and SGA, but I cannot read French. Therefore, I want to ask: 

What should I read if I want to read GIT? 

I should mention that most of my knowledge is in differential geometry. I want to read Mumford's work to understand stable bundles and moment maps from another perspective; different from differential geometry. 

Any suggestions for alternatives to Mumford's work will also be acceptable.

 A: Before I started reading/referencing Geometric Invariant Theory by Mumford, Fogarty, Kirwan I read Dolgachev's book Lectures on Invariant Theory.  I also like Peter Newstead's book Introduction to Moduli Problems and Orbit Spaces.  Given your stated interest, these treatments might be sufficient.  Even if they are not, you should be able to read them after going through Hartshorne and then I would guess GIT will be "easier" to read (the technicality will still be the same, but your intuition will be better).
Addendum (given OP's comment):  As far as a prerequisite for GIT itself, I don't know (beyond Hartshorne).  I think when you come across a concept or term you are unfamiliar with you need to just "bite the bullet" and find a reference for that definition/concept and think about the concept.  So you can use EGA/SGA as referenced directly in GIT or you can perhaps find what you need in the StacksProject (see this MO post about the pros/cons of this).  But if you have first read, say Dolgachev's notes, then when you are trying to put together these new ideas into a coherent whole it will be easier since you will have some sense of what the bigger picture looks like.
