Talk to Valaste in your starting town (vhukel guard (AD), davon's watch (EP), or daggerfall (DC), and that starts the questline, then come back and you'll get the next ones from Shallador.
If you mean the skill line, then that's by collecting lorebooks (blue glowing ones) from the overland.
Grand Master crafter, all styles and all furnishing plans known pre U41.
Vamp and WW bites available for players.
Shoot me an in-game mail if you need anything, happy to help!
You start the questline by talking to Valaste (Or someone stood outside a Mage Guild who directs you to Valaste). After that initial quest, you will receive all other quests from Shalidor who will appear when you approach any Mage Guild.
Mages Guild questline has a requirement for certain levels in the Mage's Guild in order for the quests to become available.
The final quest requires level 5 in the Mage's Guild which is the highest you need to complete the entire thing.
On a side note, same is true for the Fighter's Guild questline, it too has a level 5 Fighter's Guild requirement for its final quest.