>csc HelloWorld.cs
This is the basic command you will use on a windows computer from the command prompt to compile a simple C# console appication. csc calls the C# Compiler. In order to compile your HelloWorld program using csc you must add the location of the csc.exe to your system path.The location of csc.exe depends on what version of .NET you have installed and what version of the compiler you want to use. To find csc.exe navigate to %windir%\Microsoft.NET\FrameWork\
Select a version to find the location and you will see the C# compiler.
Useful Links
Command-line Building With csc.exe