If your flash does not have a TTL multiflash terminal, you will need a AS-10 TTL multiflash adapter. AS-15 has only one contact point and is not TTL capable. The use of AS-15 is easy. See the middle image below. Hence, you need a PC Cord to connect your flash and AS-15.
Most flashes come with a PC cord.
If yours does not have a PC cord, you can use the SC-15 . Note that SC-15 is not TTL capable. Use the ready light on the flash instead. The camera may not be able to set its shutter to the synchronization speed. Use Nikon Speedlight flashes or Nikon AF compatible flashes.