Get Minecraft. Yeah, I know, this is kind of self-explanatory, but, you'll need the minecraft.jar file in some of the next steps.
Get the MCP (the Minecraft Coder Pack). Current releases are at http://mcp.ocean-labs.de/index.php/MCP_Releases .
Get the Java SDK at http://www.oracle.com/technetwork/java/javase/downloads/index.html . You'll probably want Java SE.
Set up a system path. For windows, go to Control Panel->System->Advanced System Settings->Environment Variables. In User Variables, add the following path:
Please note that your path to the jdk may be different depending on your version of windows.
Extract the zip containing the MCP.
Get your minecraft.jar. To find it easily, open Run and type in %appdata%. It should open up a folder containing lots of folders, one of them being ".minecraft". Open that up and to go the bin folder. Within (if you've played Minecraft before) should be minecraft.jar.
Copy that jar into the jars folder of your MCP folder. As a side note, you can also copy the minecraft_server.jar if you have it. It's available somewhere on the minecraft website.
Run decompile.bat in the MCP folder.
Have fun modifying the files in sources/minecraft folder!