If you haven't already, make sure to install JDK 8. If you're using a mac, I recommend you use this JRE by Amazon, as it avoids many issues that you may encounter.
For this tutorial, Jetbrain's IntelliJ is recommended, but any IDE should work fine.
Although it's not needed, it's highly recommended that you get the MinecraftDev Plugin for Intellij, as it can do cool things like autocompleting method bytecode signatures and letting you breakpoint on mixins.
I also recommend getting a comfortable theme and background, as you will most likely be debugging errors for some hours.
Most importantly, a basic understanding of Java. I highly recommend w3schools, tutorialspoint, and MOOC.fi 's tutorials to learn java.
Note:
This tutorial was made to be more oriented toward Mac users.