As a human, you are the most populous and enterprising race, and your commerce and culture dominate the others. Half-elves living in human communities share these benefits. If you are a half-elf living with elves, see the writeup on Elves instead.
You likely lived near Ptolus, in a small village within a few days journey. The spire would always have been visible to you in the distance and was part of the draw that finally brought you here. In your home community, nearly any background is suitable to you: smith, sage, acolyte, soldier, even folk hero. Or, its possible that you've come from much further away, seeking Ptolus as a place of refuge or fortune. Perhaps your homeland was overrun by barbarians as the empire crumbles at its edges.
As a human adventurer in Ptolus, you have near unlimited options for what you could accomplish. You can seek to become magistrate or wealthy shopkeeper, a legendary delver, a Captain of the City Watch, a Knight of the Pale, a member of the Inverted Pyramid, or the leader of an influential guild or religious order. You might try to research, preserve, and advance technology. Or simply get rich and retire to the Noble's Quarter.