You'll need to download the Mod Development Kit (Mdk) from http://files.minecraftforge.net/ Unzip that, and open a command line there. Then type "gradlew setupDecompWorkspace eclipse". It'll create an Eclipse workspace in the "eclipse" subdirectory.
After that, you go ham And if you get stuck, drop in the #MinecraftForge espernet IRC channel
I would recommend using Mcreator if you don't know how to code. Just with a few buttons and texture/model imports, you can create almost anything you want! Download Here
Bad advice. With MCreator you can create simple "more blocks and items" mods. If you try to do anything complicated you will have problems, and no-one will help because MCreator produces appaling code.