I pirated this game years ago because I heard how good it was and it was so hard to find a copy of it. I fell in love with it and beat it and decided to buy it now that it's on GOG. The first time I played through the game I was a fighter, but I want to try the mage route this time.

I just made my way out of the mortuary and I'm currently level 3. My stats at the moment are:
9 strength
18 intelligence
15 wisdom
13 dexterity
9 constitution
12 charisma

What stats should I focus on advancing if I want to be a mage? I know this game has a lot of stat-boosting items later on but I don't remember them very well. I'd just like a general idea of what I should be aiming at. I don't remember all the details of the story as I haven't played it in a long time and I don't want to use a walkthrough to make sure I finish every single quest. I'd rather play it at my own leisure.

Thanks.

This is not a complete answer, but as a Mage, you can apparently do a great deal for your Charisma with the Friends spell if you anticipate a need to speak to others.
Wisdom, of course, is always fun. It may significantly improve your chats with you magely mentor as well.

When your Int is high enough, there will be no limit on the number of spells you can put into your spellbook.

Wis is quite important, especially if you want to enjoy the story -- high Wis usually gives you the most dialogue options. Similar works for Cha.

I'm still pretty fresh out the mortuary right now and I'm doing side quests and things to get XP. I intend to use a full party as I love the characters in this story plus I'd like to have some other people to take the damage for me if I'm going to get dominated in close combat as a mage.

Don't raise intelligence further, 18 is already perfect. Get wisdom to 19 next. Afterwards just raise what you want.

I just got done with the mage quests and just now got assigned to the class. Just to clarify, the higher the level I am, the more slots for spells I have to memorize? Also, will I have to buy most of my spells from shops or will they come from quests? And do the levels of the spells depend on the spell alone, or by using them do I upgrade their level?

Thanks for all the tips.

As you level up, new spell level slots will unlock.

Each spell is at a certain fixed level, and requires to be memorized into an appropriate slot before use. The max spell level you can use is roughly equal to half your caster level.

You memorize spells by placing them into the (preferably empty) slots and then resting.

You will get spells from shops, but I think also in the loot or even as some quest or dialogue rewards.

Some of your strangest, most fascinating spells will be found in dungeons or as the result of quests.
My favorites, mostly minor and useful tweaks on existing small spells, are from the Unbroken Circle of Zerthimon, which is understood through the teachings of a githzerai named Dak'kon.

I was actually wondering about those. I think I have all of the zerthimon spells up to the 5th level of the circle but the descriptions don't really say what the spells do. Any descriptions?

The only Zerthimon spell I remember is Missile of Patience. At low levels, it does 1 damage. Starting at level 8, it does 2d10 damage. At level 11, it summons a stationary gun platform that will fire for 2d8 damage at your target, AS FAST AS YOU CAN CLICK, for 15 seconds. You can level whole rooms with that thing.

Have you tried right-clicking the spell's icon in your spellbook? You need to get in there to memorize a spell anyway, but I reckon I usually press W to get to it.