Quote from CarpetClam
A lot of modders use MCP or Minecraft Coder Pack to mod. It decompiles the class files into editable java files.
Their homepage can be found here: http://mcp.ocean-labs.de/index.php/Main_Page
Latest MCP (MCP version 4.3 | Minecraft version 1.7.3) Download link: http://www.mediafire.com/?03d94f13c9ulj5a
EDIT: Sorry, got Ninja'd
Hey Thanks both of you! You guys were fast I can't wait to check it out.