Your first option is Microsoft Visual Studio for Mac which was released in 2017. If you’re used to VS ide then I suggested you download this.
If not then you can have a look into MonoDevelop. You can download from here
MRE = Mono Runtime Environment
MDK = Mono Development Kit.
MDK = MRE + Extra tools, libraries, .NET PCL (Portable Class Library) profiles, etc.
If you have an application that you want to run that needs Mono you can install just the MRE.
If you are doing development, writing C# applications, whilst you can sometimes get away with just having the MRE installed, you may hit some missing features such as not having the .NET PCL profiles. So I would install the MDK if you are doing development. Reference