You are already halfway there since you already know Visual Studio and C#! The SITCore Experimenter Kit would be the ideal starting point.
SITCore is a family of chipsets and modules that allows companies to build IoT and embedded devices using .NET C#. Coding and debugging is done through Microsoft Visual Studio. The .NET runtime environment is called Common Language Runtime (CLR). SITCore runs a “tiny” version of that runtime, suitable for tiny devices. This tiny version of the CLR is called TinyCLR!