You’ll be assigned to a Watch Sergeant who knows the ropes.
For your first eight hour shift you'll be guarding the Queen, so she doesn't die on us and throw things even further into disarray.
No promises that you'll be on guard duty for the long run, and also no promises that you won't be.
But after that I mostly expect you'll be asked to patrol the Vault. Eight hours on, twelve hours off. Maintain the peace, respond to emergencies, investigate crimes, issue warnings and - gods willing - issue citations.
At dawn I'll set up a cage to hold prisoners, which will make the job easier. You should be liberal dumping people in there to cool off. Including nobles. If they piss and moan about it you can say it's good for their Law.
But not including spellcasters, unless it's absolutely unavoidable, and if you do jail someone important get me so I can smooth things over. Don't pick fights it'll be costly for the Guard to win.