You first need to have a level 30+ fighter. After reaching level 30+, keep doing quests in Gavada to build up fame until the "Test your skills with Senel" quest is available. Take this quest, and defeat Senel. Note - It is nearly impossible to defeat Senel at level 30. Train your fighter to level 35 or higher before fighting Senel. After defeating Senel, report back to him. You will be rewarded for the quest. Afterwards, talk to him again. Senel will give you the Ninja Invitation. This allows you to change your class to Ninja. Note - It takes 1000 grade points to change to Ninja.