Open jobs
Anyone can walk in and start these: delivery, taxi, bus, refuse, mining, fishing, and hunting. Find the job's NPC and take a shift.
Whitelisted jobs
Police, EMS, Fire, government and some businesses require an application. These are handled through the Applications section of your dashboard, not in game.
Changing jobs
Your current job shows on your dashboard. Quitting a whitelisted job means reapplying, so think first.