The tl:dr of this is basically: buy equipment which benefits spellpower and spellcasters, get the amulet of transference from the Old Forest. Focus your stats on Willpower and Agility, and either presence as a support or toughness/cunning for if you aren't. Use either support powers or damage powers as a white mage, and don't use Smite Evil but you can use Charge Weapon if you use a Metal Wand. As a black mage, use grease and fireball, and either a fire summon or Vitality Reap as your encounter power. For companions, best composition is a tank, a support, and a damage dealer, though some characters dip into two categories at once.
As for what I have going on, I prefer either Black Mage or White Mage, I have saves for both. Statwise I try to even it out, but willpower is typically high for both. Equipment-wise I usually just use what I find in story or in drops, I don't typically buy anything unless it's for a quest. Power-wise again I usually just go with what I learn through tomes and leveling.
Okay, so, unless you happen to have found the Witch's outfit in the first forest area, you probably should buy some equipment. Personally I'd say you should at least have the Leather Coat from Leorah's store, it's basically a sidegrade to the Witch's outfit except it has more evasion and no bonus resistances to magic. That said, since you've beaten the Palace of Ice dungeon, so you probably have the Royal Leathers, and that's a good choice too. While some equipment is more specialized, the game is designed such that you can beat it using equipment that you can buy in Hawkthorne, so buying equipment to fill empty slots is probably a good idea. If you don't have the Amulet of Transference/Union, and it is because you haven't picked it up, there's a small puzzle section called the Ruined Shrine where you can find it in the Old Forest. Just exit Hawkethorne by going up and then just keep going to the right, and you should find it. The Amulet gives 8 spell penetration, which counters Ward, the stat that lowers magic damage dealt when hit, and can be upgraded to also give 5 spellpower, which is your main stat since you run spellcasters.
I assume you beat Tollus in the tutorial, so using the Bladed staff is a decent idea, as it is just a quarterstaff but stronger, but in terms of better spellcasting equipment you have options which are good. The first is to get the Spiraled Staff, which you can get by doing Berwyn's recruitment quest. If you haven't done it, he's the Summoner enemy in the Hill area to the right of Hawkthorne, and above level 3 you can access his quest after beating him a few times. Once you enter the quest, interact with the bookshelf to get to choose between the Spiraled Staff and the Spiraled Blade. As you're a pure caster, rather than a half caster, pick Spiraled Staff. It has the highest Spellpower and Spellpenetration of any single weapon, and as such is a great choice for pure casters.
That said, the other choice for a spellcaster is to dual-wield. The highest spellpow and spellpen you can get on a combination of two weapons can be obtained via the Kitsune, but you do have to give up the Amulet of Union to access it. The second highest needs you to buy either the Blank Scroll or the Fox Jewel from Kohaku, the Kitsune merchant, and the Metal Wand from Svern, the merchant who's part of the traveling Caravan. I personally recommend the Blank Scroll since it provides an accuracy buff too. That said, you don't need to hyper-focus on those two stats, so instead of getting the kitsune things, you can instead buy the Lynx Totem from Leorah or the Mage wand from Ivris, which provide better evasion and accuracy than the Kitsune options in exchange for lower spellpower.
Speaking of Leorah, she sells the Wizard Ring, which gives 5 spellpower. Useful for a mage.
Now then, on stats. Do not evenly spread it out. Strength is mostly worthless if you're not using melee weapons as your source of damage. Presence's main use is for tease-centric builds, though it can have use as support as it gives buffs to the damage of your allies and strengthens the stats of your summons, so if your White Mage is support-focused rather than damage focused, you may want to level presence (and also have that character learn to summon). Your black mage can also do presence if you want to have them summon, but if they're damage focused (since they're a black mage) it would be better to level any other stat to boost either their damage capabilities or their survivability, as the more damage you deal, the more likely enemies are to attack you (this is the threat system).
Aside from focusing hard on willpower, I'd say you should probably also focus on agility, as it increases both accuracy and evasion, meaning you can hit more often and get hit less often. Cunning is also a good choice, as it makes you less vulnerable to magic attacks and strengthens your crit, though you should know that accuracy (provided by agility) also makes you more likely to land crits on top of landing normal hits, and magic attacks tend to be less common than physical attacks. Toughness is entirely about survivability, giving you more health and physical resistance.
So, basically. Do willpower, agility, and either cunning or toughness (or alternate between both) for black mage, and do willpower, agility, and presence for the white mage if you build support, or do the same as the black mage for damage. You can respec stats at a trainer, like Sanders, Garth, River, and Ivris.
With powers, you probably learn enough through those means. As a support White Mage, you'd probably want to learn Blessing, Shield of Light, and either Great Heal or Holy Ward. If you don't know these spells, despite them being level up powers, you can learn them by training at Sanders. That said, if you want to go with a damaging White Mage, you probably shouldn't. Entropic Winds is relatively weak as an aoe, and both Charge Weapon and Smite Evil are weapon based, and as such are more useful on a split damage build that uses both magic and physical damage, like a spellsword or a paladin. That said, if you use the Metal Wand, your attacks use spellpower rather than strength, so Charge Weapon could be useful as a strong damage buff for basic attacks, though you can't use Smite Evil with it. Celestial Smite is good single target damage.
For the Black Mage, you should focus on doing damage. As Burner said, Grease and Fireball is the bread and butter combo of damage mages. Grease stuns all enemies and makes them more vulnerable to fire damage, and Fireball damages all enemies, even if they succeed an evasion check. I'd suggest using Vitality Reap as your Encounter Ability, though you can still use a summon if you want a distraction/damage sponge. Of course, you could also use a fire summon (either Flame Spirit or one of the upgraded Flame Spirits you can get at the Temple of Terrestrial Fire in the Frostwood area), because fire damage synergizes with grease.
In terms of companions, neither should bring Quintillus, as he focuses on tease/resolve damage. If playing support as a White Mage, either do not use Cait or use the Hexblade Leathers set for her. Her base set is a mix between healer and damage dealer, her Silks set is tease focused, her witch Gear set is much more support and healing focused, but her Hexblade Set is damage focused. Additionally, don't use Etheryn, as her damage is weak and her Raiment set is support focused. If playing a damage focused white or black mage, bring either Etheryn or Cait, as Cait is a mixed healer and damage dealer with her base set or a full support with her Witch's set, and Etheryn has group heal and a summon in her base set and a heavy support focus in her Raiment set. Also, if using the normal set Cait, her attacks synergize with the Black Mage as they deal fire damage. Do not bring both.
As a black mage, if you do not use a summon, you should probably use Brienne (genderbent Brint, if you let the cursed armor take its course). Brienne can get a set in the Temple of Terrestrial Fire which gives her fire damage and a fire damage summon, which will synergize with the damage bonus from Grease. If you do not wish to forcibly turn Brint into a woman, and do not use a summon, perhaps Berwyn is a good choice. His at-will is a firebolt spell, and he has both Group Heal and a summon. That summon is pretty decent.
That said, both White Mages and Black Mages could use a tank. Azyrran, Atugia, and Arona all have Guarded Stance as the at-will of their base equipment set, allowing them to serve as a tank. Atugia is the best at being a pure tank, as her abilities heal, draw aggro to her, and even give a shield to everyone that redirects attacks that they would have sustained back onto her. However, for damage to accompany that tank, you may want Azy or Arona. Arona does purely physical damage, with both aoe and single target powers in her base set, while Azy primarily does physical damage but also has access to the White Fire white mage power, which can synergize with Grease as it does fire damage along with holy damage.
Assuming you're using support white mage, and you don't want to do Hexblade Cait, well, you can't really go wrong with Brint. He deals tons of damage, both to groups and to single target. His alternate set, which can be found in the Glacial Rift, does frost damage, so it's not a good idea to use it in the Rift, but outside of the rift it can be strong. He can also sort of serve as a tank. Basic Brint gets a lot of threat by dealing tons of damage, though when below half health he also looses half his armor so that's not good for tanking, but his frost damage set has a shield and lets him recover health and gain extra threat when attacking enemies afflicted by the frigid status effect, which is what his powers apply.
If you want a damage dealing companion aside from Brint, Brienne, or Cait, you could try Arona. She deals damage, despite also having tank power. That said, the base set deals a bunch of resolve damage in addition to physical (most likely to boost the tank ability, as more damage means more threat, so enemies focus on her more), so for pure damage dealing, all of her alternate equipment sets are good for it.
