In your visual studio, when ever you open a project by default program.cs  will open.
  Static void Main()
  {
       Application.EnableVisualStyles();
       Application.SetCompatibleTextRenderingDefault(false);
       Application.Run(new Form1()); 
  }
 you can see above Application.Run(new Form1()); 
 
 Whatever your form name is type there, then whenever you are starting first this  form will run.