To start the quest you need to talk to a npc dragon in yalahar magician quarter named Tamoril it will say about the other 4 dragon bosses.
Tazhadur is the first one and you only need to kill 200 dragons to get the acess to the boss, the boss teleport is in darashia dragon lair.
Kalyassa lair is in Dragonblaze Peaks and to get access you need to open 5 chests
Gelidrazah the frozen lair can be found in okolnir and to access you need to answer 3 questions to the dragon statue next to teleport.
Zorvorax lair is in the vampire crypt to get access you need to walk in 3 places : the sun tower in ab’dendriel, the oasis in darashia and the stone circle in carlin
After kill all this bosses and getting the 4 items of each boss : tooth of Tazhadur, horn of Kalyassa, scale of Gelidrazah and bones of Zorvorax, you get acess to the first dragon boss.
For more info : https://tibia.fandom.com/wiki/First_Dragon_Quest