Hello to you too.
Wherever you want to close the application you can add code like: application.exit();
Now if you want to do this by closing a specific form you have to add in form's close event [application.exit();].
About the old and new form : All forms are objects.When you create a project visual studio creates a form for you. Now if you want to make another and use it as "main" on your application you have to change the code in program.cs .
In the Program.cs class there would be Main function something like below
static void Main()
Change the Application.Run(new Form1());
This should work.
If you need any example in code just let me know.