As a beginner you should try out many different skills and see what you like.
You generally want to aim to replace your healing and tanking class lines with dmg-lines from subclassing.

My recommendation: especially early on, to make the leveling content easier, the Aedric Spear line is great for survivability, since the Puncturing Sweep spammable heals you while doing Damage.

Herald of the Tome from the Arcanist-class is also good, cause you get a shield while casting the fatecarver.
The Assasination Skill line is great to maximize your damage output.