So I've never played a Mage of any sort, at least not in Shadowrun. I'd like to, however, for a game starting in a couple of weeks. I've heard that possession mages are pretty good, so maybe I'd like to play one of those. I'm not really sure what capabilities a mage can have, but I figure there are some general points I'd like to hit - Initiative Passes, being an Elf, and balancing depth and diversity of skills.

I could use some help, of course, in building this guy and figuring out what sort of stuff mages can do. So, I guess, what can mages do, and how would I build an optimized posession mage? What tradition would be best? What sorts of spirits should I summon (if in fact I'll be summoning things), et cetera?

Oh, and I've got Street Magic and the main rulebook, what other required reading is there?

The Voodoo tradition is definitely the best possession tradition (well, in mechanical/crunch sense), since it has the most powerful spread of spirits. When you're building your character, prioritise Willpower and Charisma (for drain resistance), Magic, Spellcasting, Summoning, and Binding. Counterspelling is handy too, and Ritual Spellcasting can be awesome if opportunities to use it arise in your game (in some games they never do), but don't bother taking Banishing (just blast naughty spirits with low drain combat spells). If you want to do self-possession, make initiation a priority so that you can get the Channeling metamagic.

While Possession mages are pretty good for very specific builds they are inferior to Materialisation tradition mages in general. Make sure you read through some of the other threads here to see why. Start with this one and also check out the Tactical Manual.

Cool, I'll check those out. And what does a Materialization mage do? As far as a Possession mage goes, I was under the impression they could possess themselves to good effect, and that interested me thematically, which is why I asked.

This thread was another one where someone was looking into magic / tradition / possession vs materialisation questions.

Materialisation tradition mages can summon spirits that can appear on the physical plane (materialise) whereever they want. If it's behind a ward, it will cost you a service, but they can do it. So you are faced with a door which is locked? Just materialise a spirit on the other side to turn the handle on the inside of the building to let you in. Need an extra combatant in a battle? Your spirit can materialise and take some hits, dish out some damage, that kind of thing while you get on and heal your buddies or run away. If your GM allows it, you can even get your fire spirit to materialise inside the petrol tank of an opponent's vehicle.

On the other hand, possession spirits need to take over a vessel to affect the physical world. You can spend hours preparing one to make that test more likely to succeed, but most opponents will run away before you complete the ritual so you have to wing it and hope they succeed. Or you can lug around bulky and heavy homunculi for them to possess, but then they can't go through wards and if you need to make a getaway you may have to leave behind your expensive homunculus.

On the plus side, possession spirits possessing the summoner sacrifice an extra combatant on the field for a slightly tougher mage. With metamagic Chanelling, augmented skill set too. Sometimes that's worth it. Generalists are better off being materialisation traditions though - especially if this is your first mage.