how to start up form in c#

Look at your application's entry point (usually "Main"). It has the line:

Application.Run(new Form1());

This start of application defines what is your main form. When you close a main form, the whole application quits (unless you handle FormClosing event to cancel it).

If you want to show other forms, you need to… well, show them:

//pretty sloppy code:
Form newForm = new Form2();
newForm.Show();

//more accurate code:
Form mainForm = new Form1();
Application.Run(mainForm);
Form newForm = new Form2();
mainForm.AddOwnedForm(newForm);
newForm.ShowInTaskbar = false;
newForm.Show();

Making a form owned by a main form makes them activate together, which help integrity if application behavior. In this way, no window from other application can appear in Z order between the forms of your application. Not showing a form in a task bar complements this behavior. In this way, only a main form represents whole application it the task bar.

See: http://msdn.microsoft.com/en-us/library/system.windows.forms.form.aspx[^].

—SA