[Nuria]

En el OnClick del Botón :

Si el formulario es MDIChild .

  fForm := TfForm.Create(Self);
  try
    fForm.FormStyle := fsNormal;
    fForm.Visible:= False;
    fForm.ShowModal;  // ó fForm.Show
  finally
    fForm.Free;
  end;

Si el formulario no es MDIChild

  fForm := TfForm.Create(Self);
  try
    fForm.ShowModal;  // ó fForm.Show
  finally
    fForm.Free;
  end;

Saludos!