Is there some kinda secret
'CULT OF THE HOLY MINECRAFT SOURCE CODE'
around here somewhere? I dunno, where else would you learn how to modify minecraft, or figure out what all those damn Class files do?
It's called Google, use it.
With a little work you will find all sorts of modding related stuff, like MCP, FML, Minecraft Forge, the list goes on... Plus you will need at least a basic understanding of Java, and probably want to install the Eclipse IDE and the Java SDK. Also if you find your way to the Forge Forums don't post like you just did or you'll just get banned there.
That's all you should need for vanilla modding. However, for example, I needed
thing I made, so you would want to download that, add it to your minecraft.jar in the MCP folders, and then de-compile.
(pointless post really, just linking for the lazy)