A working understanding of Autodesk Revit.
An installation of an Autodesk Revit-based product, including the Software Development Kit.
MS Visual Studio 2015 Express Edition (C# or VB.NET). Microsoft Visual Studio 2015 Professional is recommended, though, as Express editions do not support DLL debugging. Alternatively, you can use the built-in SharpDevelop development environment in Revit.
Some experience in a .NET based development language. (Autodesk Revit API examples are provided in C# and Visual Basic.NET.)