These are just my opinions from being around the game for almost 3 years and seeing a lot of people hired and fired from Staff positions.
Keep in mind that minimum age requirements for Staff are usually around 16-18, sometimes as much as 20 years old. It mainly depends on who is asking for applications.
Play the game for at the very least 6 months to a year.
Make yourself known within the Realms as a respectful, helpful and creative person.
Keep your nose clean (don't get banned, don't annoy or cause trouble, for Staff members especially, but everyone in general).
Participate in things where you can show off your skills like Quests, the Storytelling Contest or post in the Cornerstone Inn Forum.
Post INTELLIGENTLY on the boards.
Get to know some Staff members and earn their respect. (Do NOT bug them over and over about open positions...they rarely know more about it than anyone else)
Make your desire known, but do not be annoying about it.
Volunteer to help in any way you can without expecting compensation.
WAIT for a Department Head's call for applications to go out (usually happens once or twice a year but not for every department) then apply and WAIT again for a reply...The big key here being the waiting.
Even after all this you are by no means guaranteed a position. There are many many people who've been around for years who want to be on staff and are certainly willing and qualified but haven't been hired for whatever reason. Just try to make a positive impression, and remember that good things come to those who work hard and wait for the results.