Why hello there! It's DrGravy here. This is like, my 3rd post so I better start contributing god dammit. Well this tutorial is for people who have never really touched Java programming before, or Minecraft modding for that matter. So before we start, we better answer a few basic questions.
Question: Wait what the hell are you teaching?
Answer: I'm basically making a tutorial on how to decompile Minecraft.
Question: What does this let me do?
Answer: It lets you make your own modifications to the Minecraft client.
Once downloaded, run it and go through all of that next, yes, I agree **** until it's done. Now you're going to want to navigate to C:\Program Files\Java and find the folder labelled jdk1.7.0_03 (Or whatever version is the most recent), enter this folder and you should see, at the top another folder called Bin. In the folder location at the top, it should look something like C:\Program Files\Java\jdk1.7.0_03\bin. What this is the path for your computer to look for the Java Decompiler. Copy paste that and right click on Computer and hit properties, proceed into Advanced System Settings, and then hit Environment Variables, click the button that says New, make the Variabl name 'Path' and the value is that code which is the address of your bin folder in Java. Once you've clicked OK and all, to test if this works, open up cmd and type in 'javac', if it worked, it should come up with a cluster**** list of commands. But we don't need to get into that, it just means you've done it right, which is good. If that didn't work for you, re-try and make sure you have the latest version of Java installed (Java 7).
2.) Setting up MCP with Eclipse
Aight so, you've got the compiler all set up, but no way to use it with minecraft at the moment, for this, we're going to use a wonderful tool called Eclipse. Head to this page: http://www.eclipse.org/downloads/ and download the Eclipse Classic. Do all of that 'yes, yes, I agree' **** again with this until you've got this set up and ready to run, and then close it. Because before we use Eclipse, we're going to need MCP to actually do much with the minecraft code. Head to the MCP Website http://mcp.ocean-labs.de/index.php/MCP_Releases and download the newest release. Make sure it's for the appropriate version of Minecraft you're trying to modify (and that you have the required .minecraft for that version). Unzip it to an appropriate location on your computer, and run the cleanup.bat, and then updatemcp.bat, once this is done, a few more folders should appear at the top, this is good. I presume you guys know how to access your .minecraft folder, once in there copy paste your bin and resources folders into the Jars folder in the mcp62. And finally run decompile.bat. This should take roughly 34 seconds. Open your eclipse folder in the mcp62 folder, and copy paste the directory onto your clipboard. Now it's time to open Eclipse. It will ask you what you want your directory to be, copy paste that code into there and hit OK. Let it load then click the arrow next to Client and then src,and now finally hit the small arrow beside net.minecraft.src and an asstone of .java files should appear.
Congratulations, you've just decompiled the minecraft client and you're now ready to code some hacks.
(Right after you learn how to).
Hope you enjoyed my tutorial, and found it helpful.