Select, with the approval of the referee, a background that represents your culture and previous (non-adventuring) training/experience. The selection of a character’s background is as important as the selection of a class as a character’s background gives the character a broad base of skills and knowledge.
The referee will consider your character's background just as he would your character's class when deciding if a character will succeed with an action. For example, a character with an “(Great Imperium) Engineer” background should have a much better chance of damming a creek or building a bridge over it than a character with a “Courtier” background. That character should have an even better chance if that creek is in the territory of the Great Imperium where the character knows more about the terrain and likely has contacts who could help.
Cultures are generally limited to those the referee has defined for the campaign.
For the prior training/experience part of a character’s background, anything that fits the campaign setting may be selected. For example: A few possibilities include: acrobat, alchemist, animal trainer, architect, aristocratic noble, chef, con-woman, desert nomad, goblin exterminator, hunted outlaw, knight errant, priest, refugee, scout, shaman, shepherd, soldier, spy, temple acolyte, thief, torturer, traveling martial arts pupil, tribal healer, tunnel scout, wandering minstrel, poet, and so on. This part of a character’s background need not be related to the PCs class, e.g. a player who creates a deeply religious fighter skilled in the arts of vision interpretation, divination and oration might pick 'Prophet' as a background. Backgrounds may not duplicate a class.
Remember to discuss your background ideas with the referee to be sure it will be a good fit for both the campaign setting and the group’s play style as well as to be sure you and the referee are both on the same general page as to what the background means. For example, if you picked a “knight” background, the referee might be thinking “knight of the round table” while you are actually thinking “Knight Templar.”