Getting Started (For Modders)
Note that this guide is for Windows users. If you're using OS X or Linux most of the tutorial will be applicable but specific file locations, etc... may be different.
Slay The Spire is written in Java. So you'll need some prior experience using the Java Programming Language or be willing to learn.
In order to do modding for Slay The Spire you're absolutely going to need ModTheSpire link. It's also recommended that you download BaseMod to have access to an easier modding API link.
For this tutorial we are going to be using a folder called my_mods in your Documents folder. This folder (the my_mods folder will be where you can place all your mods). For this example mod we will create an example_mod folder inside my_mods to contain all the code for this mod. 
Now inside the my_mods folder let's go ahead and create another folder called lib (lib is short for libraries). This folder is where we'll place the dependencies for your mod. Go ahead and move the ModTheSpire.jar and BaseMod.jar that you downloaded in the last step into your lib folder. 
There are two popular IDEs used in modding Slay The Spire. The more popular of the two is IntelliJ, as it is easier to set up, but you can use Eclipse if you desire.
Follow this guide to decompile Slay The Spire to (essentially) look at the game's source code.
Having problems? Check out this troubleshooting guide