In order to join the Mages Guild, seek out a local head of the Mages Guild in any of Cyrodiil's seven guild halls. There is one in each city except for the Imperial City and Kvatch. All you need to do is ask to join the guild; as long as you have a clean criminal record you will be made part of the guild.

Where do I start the Mages Guild quest in Oblivion?

Guild Halls[edit]

Mages Guild Halls are located in almost every city in Cyrodiil. In the Imperial City, the Mages Guild is found at the Arcane University.

How do I get to Mages Guild?

The only method to “unlock” the Mage's guild skill line is to find the Mage's Guildhall (available in the first major city of each starter zone) and there will be an NPC outside ready to help you join. You can learn how to level up the Mage's Guild skill line here.