The starting form is the first form that's opened. If you're using code generated by the new project wizard there will be a *.cpp file named the same as your project. That's where main() is, and in main() you'll see something like this:

C++ Syntax (Toggle Plain Text)

// Create the main window and run it
Application::Run(gcnew Form1());

// Create the main window and run it Application::Run(gcnew Form1());

Change Form1 to the name of the form you want to be the starting form and that's it!