If I understood correctly, in your Main method (where you're starting your application), first create an instance of your first form, show it as dialog and after that run the second form.
Something like:
static void Main() {
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
if (new IntroductionForm().ShowDialog() == DialogResult.OK) {
Application.Run(new MainForm());
}
...