Talents are unlocked when a player finishes the quest Talent: Strength Unlocked at level 20. After you complete the quest you will have access to the talent page on your character's profile.
You can fill your talent orb with power by using a sealed familiar on the orb. This increases how much power is in the orb based on how much talent power is given by the applied sealed familiar. Each familiar has a set amount of talent power and leveling up a familiar before using a seal stone on it does not affect the power.
You will continue to unlock talent skill slots every 5 levels and also gain access to more talent skills.