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: 



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> **What should I read if I want to read GIT?** 
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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. 



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> **Any suggestions for alternatives to Mumford's work will also be acceptable.**
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Preparation for GIT (Geometric Invariant Theory)
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](https://stacks.math.columbia.edu/) (see this [MO post](https://mathoverflow.net/q/207388/12218) 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.


