How locked am I in my first class choice?

[Duuude007]

You can have all classes and all jobs with a single character. Nothing locks you out of any job choices. You can join all of the class-related guilds. The only restriction, if I recall correctly, is that you have to reach level 10 on your first class before the Armoury (class-change) system is unlocked.

To be more accurate, you need to finish the first 3 quests at your guild to unlock the armory (means you can switch classes whenever.

Level 1 quest
Level 5 quest
level 10 quest

After that the guild leader gives you permission to join other guilds.

One thing to keep in mind is if you want to reach other cities "easily" for the first time, keep doing main scenario quests until about level 15. You will be given an airship pass then, which means you can go to any of the 3 cities to check out other guilds with ease.