You served in the guard, out of either patriotism or the need for coin. Either way, you know how to get a difficult suspect to talk.
However you left the guard, you might think of adventuring as a way to use your skills on a wider stage.
Choose two ability boosts. One must be to Strength or Charisma, and one is a free ability boost.
Source: Pathfinder Core Rulebook (Second Edition)