Which caster for new player?

As a beginner, I would favor classes whose duties are limited, with minimal micromanagement and situational switching of ability priority.

For Albion, Theurgist (41 Ice/35 Earth) is a strong choice that's desirable in RvR and PvE for its fire-and-forget pets and pulsed bladeturn. They interrupt enemy casters and supports, and incoming melees are snared. If given sufficient casting time, their pet spam overwhelms any target. Caster speed is a nice bonus, shared by my choice for Midgard - the Runemaster (47 Darkness/26 Suppression). They're powerful nukers who also have pulsed bladeturn, nearsight, and a melee damage add buff. Anyone approaching is shredded, and they neuter the enemy backline by drastically reducing their casting range. For Hibernia, the Mentalist (46 Light/28 Mentalism). They're another nuker who may charm any mob and recover from mesmerize by letting their pet attack them. They then demez allies and start pumping out damage until one side is dead.

I suggest Ice or Air while soloing as a Theurgist, earth for grouping from 26 onward. Runemaster, Darkness or Runecarving while solo, Suppression for grouping from 26 onward. Mentalist, Light spec for soloing and Mana for grouping, with the caveat that Mana's AoE DoT and kiting technique at 17+ is acceptable at soloing too, if you can handle the playstyle (I find it tedious). Respecs are infinitely free until 50 or realm rank 3, so decide on your final spec by level 49.5 or you'll have to waste your single free full respec at 50. When leveling, remember to use /train in the field, though spec changes require your class trainer.