1) Take a personality/aptitude test to figure out what kind of person you are and what you're capable/interested in doing (ie do you want to work for the govt because of a stable job/benefits?)
2) Consider that many people graduating with a liberal arts degree can point to basic skills such as persuasive argument or excellent writing skills. A degree in that sense can be used to convey not so much an area/subject of expertise than an acknowledgement of specific skills mastered.
3) Besides focusing on your skills/abilities, you can also say that History has equipped you with a way at viewing the world ie a philosophy or methodology of approaching viewpoints or a problem. I remember reading in a book by Donald Trump an anecdote about a female who said she could predict market changes by listening to "ping pong" noise which she applied from having played ping pong in her youth (subtle angles of a racket make different noises when hit by a ball).
4) The process for applying for govt. jobs varies...for starters, you can go to
http://www.usajobs.gov/ which is a monster.com-run site for the federal govt. Jobs tend to be either open/competitive which means that they're open to the public provided they meet certain requirements or can be closed/noncompetitve which I take it to mean that it's restrictive (ie limited to agency/govt employees). Perhaps someone in govt such as marlin can further elaborate on govt jobs?
A civil service exam as I understand it is a standardized exam given to people applying for a government job (fed, state, city etc). It seeks to test basic skills, aptitude/fitness for the job or whatever is legally acceptable in determining a candidate's ability and compatibility for the government job opening. It also puts people on a list based on their test score and ranked, usually with people who scored higher given priority for the job opening.
The only other thing I can say at present is that some states/cities/towns etc. have their own websites for job opportunities. When I applied for a job at the NYC, for example, I went to
http://nyc.gov/html/dcas/html/employment/dcasjobs.shtml to view the list of available jobs and see what the requirements, duties, and application process is. Occasionally you may find a town announcing an opening in the newspaper's classifieds.