When you are Level 20, speak to Grand Master Tobias (Gludio) to become an Assassin. By l2hub
Steps from the game client
Towards the Altar of Rites
Quest that must be fulfilled to change occupation to Assassin. To become an Assassin, Triskell asks you to prove your skills by helping Witch Arkenia who can be found at the Altar of Rites.
In Search of Shilen's Tears
Witch Arkenia asks you to recover Shilen's tears that were stolen from her. You can get more information about the theft from Guard Leikan in Gludin Village. Go and see him.
Leikan's Request
Guard Leikan tells you that if you obtain 10 moonstone beast's molars for him, he will tell you who stole Shilen's Tears.
Guard Leikan tells you that the thief who stole Shilen's Tears is an orc named Calpico. You will find him near the School of Dark Arts. Kill him and recover Shilen's Tears.
You have returned Shilen's Tears to Witch Arkenia for which she gave you a Recommendation to become an Assassin. Take it to Triskel in Gludio Castle Town.