+ 2
It can't compile and execute your program because the compiler needs a Main method. No choice!
That's true for many other languages, like in c++ with int main() {}
+ 2
In addition to Theophile, you need a Main method to give the application an entry point.
How did you start in Visual Studio ?
You need to create a c# project, you cannot start by using a single class.
Try the code you have writen in the sololearn playground.
Do you want to use winforms or wpf.