When visual studio is compiling it is looking for this
public static void Main()
// start up code here.
in order to know where to start executing your program - if you have more than one of these, then it doesn't know where to start and throws an error.
You have two of these somewhere.
This is commonly caused by not starting new projects or solutions when you are learning C# and instead just adding new files to an old project. In the future just make sure to start a new solution (.sln) for every class lesson or project.