What do you download to use Java?

n1nja2na Once you compile the code this will convert the Java to code your computer can understand.

FYI, compiling is converting to a file that a computer can understand. You code, code, not compile:)
Everything else +1

You're very welcome.

So the first thing you'll need is the JDK, Java Development Kit. Once you compile the code this will convert the Java to code your computer can understand. After installing the JDK, get Eclipse. Eclipse is a nice software for coding.

For the JDK, http://www.oracle.com/technetwork/java/javase/downloads/index.html
Go down to where it says, "JDK", and download it.

For Eclipse I think it's this,http://www.eclipse.org/downloads/packages/eclipse-ide-java-ee-developers/keplersr1

This is all the software you'll need to get started Java programing, but for starting Minecraft Modding, I recommend you use MCP. http://mcp.ocean-labs.de/download.php?list.2 I'm not sure which to use as I haven't done any modding for about a year. I'm more of a c++ kinda guy.

Hope this helps, remember to use lots of tutorials.

Everything you need to know here! http://www.minecraftforge.net/wiki/Basic_Modding

Once you progress enough into the tutorials there, you should search for other sources of modding tutorials as these can be pretty basic.