Ideally you should be able to get your job soon after your main class gets to 30. Some people get to 30 and then go level the second class. Some people level the second class first. Some swap back and forth while they are leveling both. Whatever you feel like.
The main point is that people in dungeons will be expecting you to have your job skills by the time you get to Titan (level 35).

Each class levels separately from level 1. All bonus attributes are individual by class - so those points from conjurer will disappear when you switch to arcanist. The arcanist class will earn their own bonus attribute points as that levels up.
The job is like an add-on to the base class. The job level is always the same as the base class level - the earn experience at the same time.

Speaking of benefiting your group play as much as possible, you will also want to get thaumaturge up to level 26 at some point for swiftcast.