Hi Xanaa,
The C= RGB to 15 pin adapter is just a gender changer. The only way you will get a picture on the LCD screen with this adapter is in workbench using the VGA monitor driver. But again only in workbench will you see an image, games and early startup up menus will not display anything.
You will need a scandoubler or visit Red's store for an IndiVision AGA, or Amigakit.