Mages Guild

You can "join" the Mages Guild at any of the local guild halls in the seven minor towns around Cyrodiil. Ask around in each guild to learn who you need to speak to become an associate of the Guild. Before you become a full-fledged member and are permitted entrance to the Arcane University in the Imperial City, though, you'll have to complete the recommendation quests for all of the local branches. You can do them in any order, though.

If you played Morrowind, then you may be quailing at the thought of having to get your skills and attributes up to a certain level to advance in the Mages Guild, but fret not: there actually aren't any skill/attribute requirements for the guilds in Oblivion, so even a hearty warrior will be able to become Arch-Mage of the Mages Guild, provided he can cast a few simple spells. (Or can afford to buy scrolls that let him pretend to cast spells.)

Recommendations

Anvil Recommendation

Things can get pretty complicated when dealing with Caminalda.

Speak to Carahil in Anvil to earn this quest. Your task is to find a rogue mage, who's been killing merchants as they travel along the road outside of town.

Travel to Brina Cross Inn and speak to Arielle Jurard, a battlemage that's been sent by the Mages Guild. She'll get all cloak and dagger and ask you to rent a room for the night, so do so. Booty call! Be sure to tell the innkeeper that you're a merchant.

Head up to the room and rest for an hour to get Arielle to come on up. She'll lay out the plan for you: you're to leave the Inn and start travelling along the road while Arielle and another battlemage shadow me. When the rogue mage attacks, they'll step in and finish the guy off. Sleep for one hour, then follow the plan.

And...well, for once, the plan is actually followed to the letter. If you travel along the road for a bit, you'll be confronted by another resident of the inn, Caminalda, who likely spoke to you after you told everyone that you were a merchant. She'll get right up next to you, so feel free to attack her as soon as she confronts you. If you suspect that she'll take you out, just wait for Arielle to come along and lend a hand. Caminalda will die soon enough, allowing you to return to Anvil to complete the quest. (You can also take the key on her body back to the Brina Cross Inn to unlock her door and the chest within.)

Skingrad Recommendation

Rescuing Erthor will net you your Skingrad recommendation.

Speak to Adrienne Berene in the Skingrad guild for this quest. She wants you to track down Erthor, a fellow mage in Skingrad who borrowed some notes of hers. Unfortunately, he seems to have gone missing, so it'll be up to you to find out where the heck he's been.

Speak to Sulinus Vassinus downstairs to learn that Erthor has been hanging around in Bleak Flats Cave where he's been practicing his magic. Unfortunately he doesn't remember where it is...no problem, though, because Druja, the Argonian mage in the Guild, does. Speak to her to gain the map marker for Bleak Flats, then speak to Adrienne again for a free Weak Fireball spell.

Bleak Flats Cave is north-northwest of Skingrad, so follow the compass pointers until you reach it. After heading inside, you'll discover that it's full of something called "deranged zombies", presumably the result of a failed experiment of Erthor. Zombies are weak to fire magic, so feel free to blast them with the Fireball Adrienne gave you. When you reach Erthor, he might explain what's going on if you speak to him before killing all of the zombies, but in our case, we happened to clear the cave out before speaking to him, and he acted as if the quest was already done.

In any case, when all of the zombies in the cave are dead, you can apparently escort Erthor back to the entrance and return to Skingrad.

Chorrol Recommendation

Speak to Teekeeus in the Mages Guild here to get his quest. He wishes you to speak to one Earana, a rival mage of his, who's suddenly appeared in town. They once had a spat at the Arcane University, and apparently managed to get each other kicked out somehow. Teekeeus is apparently a stickler for the rules and regulations of the guild, while Earana is more of a free spirit; you can see why they'd get on each other's nerves.

Teekeeus wants to find out why Earana is in town. Before you go out to interrogate her, speak to the rest of the Chorrol mages to learn more about the pair and their history. When you're ready to track her down, head over to the Gray Mare Inn to speak to her. It turns out that she's not here for Teekeeus at all: instead, she wants you to find a book for her. That doesn't really explain why she's in town, but hey, you are extraordinarily good at finding things and bringing them back, aren't you?

Ignore Earana's warnings about speaking to Teekeeus and tell him of the conversation. He'll give you a map marker for the location of the book, and tell you to get it, just like Earana. Teek here, though, wants the book to be in the Guild's safekeeping.

Earana seems to have an abrasive personality.

Warp as close as you can to Cloud Top and walk the rest of the way there. It'll be difficult to make it up the mountain from the south, so you may want to either approach from the east, or bring a horse along to help you make the ascent. When you find the compass marker, you'll notice a burned corpse on the ground; search it to find the book.

Earana or Teekeeus? Now that you have the book in your possession, you have a number of options. You can give the book straight to Teekeeus if you just want to obtain your Guild recommendation, but that's kind of anticlimactic. You can give the book to Earana if you wish to obtain a special spell, but that will prevent you from gaining the recommendation, won't it? Dilemma.

Well, luckily for you, there's a way for you to obtain the unique spell from Earana, and get the guild recommendation from Teekeeus. To do so, begin by giving the book to Earana; she'll ask for a day to translate it. Wait around, or go questing elsewhere, or whatever. When a day's passed, return to Earana and talk to her. (If she tells you to go away, give it a few more hours, then try again.) She'll give you the Fingers of the Mountain, Part II quest which, when completed, will net you a unique Shock-damage spell. (To complete the quest, grab a Welkynd stone from an Ayleid ruin, then return to Cloud Top and cast any kind of Shock spell on the unique-looking pillar there. You'll have to survive the shocking damage that's returned to you, though, so you'll need some kind of shock resistance on your person.)

But hey - Earana doesn't give you the book back! Teekeeus is naturally upset by your choice of loyalty, but it's all good. If you break into Earana's room in the Gray Mare Inn (the one at the end of the hall), then pick the lock on her treasure chest, you'll be able to lift the Fingers of the Mountain book and return it to Teekeeus for your recommendation.