The Wizard's Guild
Ok, on to the Wizard's Guild- my personal favourite. You put in hours of hard work, desperately trying to cast spell X, only to learn that the spell is nowhere near as good in practice as the nonmagical parts of the Disc would have you believe. However, despite the struggles involved, I suggest that anyone looking for a challenge attempts a wizard character. They can be great fun, and (Wizards) is a great talker channel. Ignore what you've heard about Wizardly Whining (or Whinging), and see what magic can do for you!
1. Joining the Guild
You can Join the Wizard's guild at many of the main locations- Two that I'm sure of and one that I assume works. You can join in the upper west stairwell of Unseen University (UU), and also at the chancellor's office in the Institute of Illusory Learning (IIL). Also, I assume the Wizard's Guild in Bes Pelargic will allow Agatean would-be wizards to join. However, I am not at all sure that one can join at the guild in Sto Lat.
2. Life as a Newbie Wizard
Those first few (many) levels as a wizard will be a total pain in the arse. I suggest you find things you can kill easily, and commit some friendly genocide until your fighting skills are at least bearable, maybe in the 50+ range. (Level, not bonus!) Slowly work your way up the chain of killable things until you have a decent XP income- at least 50k/hour- I'd suggest 100k/hour. Now, build up your primary magic skills, focusing on magic.spells.offensive, magic.spells.defensive, and some of the magic.methods skills. Once you reach Guild Level 40-50, you can pick which order you will join. This choice is permanent. There is no going back. I won't cover the orders here, but each has a different skillset. You can find information about them at the Discworld Homepage.
3. Profit!
At this point, choose either to advance magic to do your fighting for you (not reccomended...yet) or keep advancing your fighting- and your magic.spells.defensive. Defensive is an essential skill for any wizard, guildmax it as soon as possible.