SHORTTUTORIAL: The Basics of Modding - roxxploxx/RimWorldModGuide GitHub Wiki
Levels of Difficulty
Easy: Create XML to apply existing functionality in new ways to make new things
Easy if can code: Create a C# class to make new things. These c# classes can be used in XML ThingDefs.
Harder: Write C# classes to extend Rimwork and use Harmony in HugsLib to modify existing code to do brand new things. This is the bulk of what is in this wiki.
So you want to...
Add a new Thing to RimWorld - Add a new ThingDef in XML
Add a new Thing to RimWorld, like an existing one - Read the Core XML and use one if the abstract XML definitions as a Parent. You'll inherit it's properties.
Add a new building or plant - As above, but make sure the category tag is set so your building tag gets used.
Create new functionality - If it doesn't exist, create a C# class to do it.
Create new functionality of a Thing in RimWorld - Create C# object and a ThingDef that uses it with <thingClass> pointing to MyNamespace.MyNewClass.
Add new data to a Thing in RimWorld - Create a new ThingDef class and use that in your XML as 'Class="YourThingDef"'