You should use a main class and call all of your forms from that, so
something like this:

public class clsMain
{
    public clsMain()
    {
        //
        // TODO: Add constructor logic here
        //
    }

    [STAThread]
    static void Main()
    {
        frmSplash objfrmSplash = new frmSplash();
        objfrmSplash.ShowDialog();
        clsGlobal.g_objfrmMDIMain = new frmMDIMain();
        Application.Run(clsGlobal.g_objfrmMDIMain);
    }
    //This is the Single Threaded Apartment Model(out of our scope) of the
application which will run the Main function
    //when the application starts
}
on your splash screen form place a timer that closes the form after a
few seconds.
(Note:) dont forget to remove the main class from the form that may
already be your main form.

/// <summary>
/// The main entry point for the application.
/// </summary>
//[STAThread]
//static void Main()
//{
//    Application.Run(new Form1());
//}
//because we cannot have two Main function.We are invoking everything
from clsMain