To develop IntelliJ IDEA, you can use either IntelliJ IDEA Community Edition or IntelliJ IDEA Ultimate. To build and run the code:

• Run getPlugins.sh / getPlugins.bat from the project root directory to check out additional modules.
• Install the Kotlin plugin from the plugin manager (version 0.12.613). Parts of IntelliJ IDEA are written in Kotlin, and you need the plugin to compile the code.
• Make sure you have the Groovy plugin enabled. Parts of IntelliJ IDEA are written in Groovy, and you will get compilation errors if you don't have the plugin enabled.
• Make sure you have the UI Designer plugin enabled. Most of IntelliJ IDEA's UI is built using the UI Designer, and the version you build will not run correctly if you don't have the plugin enabled.
• Open the project.
• Configure a JSDK named "IDEA jdk" (case sensitive), pointing to an installation of JDK 1.6.
• Unless you're running on a Mac with an Apple JDK, add <JDK_HOME>/lib/tools.jar to the set of "IDEA jdk" jars.
• Configure a JSDK named "1.8", pointing to an installation of JDK 1.8.
• Add <JDK_18_HOME>/lib/tools.jar to the set of "1.8" jars.
• Use Build | Make Project to build the code.
• To run the code, use the provided shared run configuration "IDEA".

You can find other useful information at http://www.jetbrains.org. Contribute section of that site describes how you can contribute to IntelliJ IDEA.