To change class, you first need to finish the level 10 conjurer class quest from the guildmaster.

Then you can visit any other guild and sign up to learn that class. You won’t lose your progress as conjurer, and can switch between them at any time when you’re not in battle (and not in a dungeon or instance).

To become a scholar you’ll need to start as an arcanist. The guild is in Limsa, to the west of the market alley.

To upgrade from arcanist to scholar, you’ll need to finish all the class quests (up to level 30) and reach a certain point in the MSQ. (Join a grand company and get sent to investigate the sylphs.)

The astrologian guild is in Ishgard, which you can’t access until you’ve completed all of ARR and started the Heavensward expansion. The only way to get access to the job before that point is to pay for a level boost book from MogStation, which will send you straight to Lv60. (It will also mark all the job quests as complete, so keep that in mind if you don’t want to miss out on side stories.)