I would like to start using C# in Rhino but I do not know where to start from. Can someone point me in the right direction and tell me which programs I would need in order to start writing my own plugins in C# and maybe a source of good tutorials? (I have experience scripting in Python but I am a beginner to C#)
Rhino doesn’t supply a programming environment for C#, so you will need to get Microsoft’s Visual Studio. The full Visual Studio is a commercial product that come at several price and capability points, but a perfectly usable version for Visual Studio beginners is called Visual Studio Express and it is a free download. If I remember correctly, there are several versions of Express based on which language they support: C#, Visual Basic, C++, etc.
When writing Rhino plugins, the Visual Studio environment needs to be set up to run Rhino as the debugging startup app; when this is done correctly then whenever you start a debug session Rhino will load and your plugin will be available in Rhino as a command. Your plugins will need to import RhinoCommon, of course, to use the Rhino functions.
This is just the sketchiest of overviews, but hopefully will get you started. Presumably you have already had a look at McNeel’s Rhino developer Wiki?