Before you can go on duty you need a mentor, they will teach you the commands, the rules and how to enforce them properly and the punishments for rulebreaks. If you need one then message @Turtle or @Florian on the forums.
When the seller has made the form you will gain access to the job whenever roast has done a custom job restart. You can check this in the Discord. Once you have the job you can switch to it via the f4 menu.