Before you can start modding you need to have the Java Developers Kit installed.
You might not actually find a direct using when you are coding for Minecraft, but without it you can’t even install Forge.
The first thing that you have to do is watch this video and do exactly as he sais. Else it will not work at all.
After installing JDK you will need Eclipse.
There are several versions of Eclipse on this page. The one you want is called: Eclipse IDE for Java EE Developers.
Eclipse is the program that you will use to write your code, you can use to test, lets you read Minecraft code and will help you with the coding.
To set this up you have to download the Eclipse file, extract it with 7zip or winrar and run the eclipse.exe. That’s it for this tutorial. In the next one I will teach you how to set up mcp with forge and start eclipse in the right map.