Here are a few things that you should know as you start your film career, things that aren't discussed very much. I wish I'd known them when I was starting out.

Everyone's a freelancer. 

Unless you run a studio, you're essentially living gig to gig.  Even development staff, equipment rental house grunts, attorneys, soundstage managers, and lab/post house staff are all one cancellation away from not having work.  So...

Don't stop learning. 

The last time I counted, over 60 video formats - both tape- and file-based - were introduced since the 1960s.  If you threw all the obsolete decks, cameras and accessories into the ocean you could create a sizable island, which you could then populate with all the people who got to know one technology and never adapted to the next one.  Don't be on that island.  Keep learning.  New technology permeates every aspect of this field, and the people who can master it without going broke doing so will stay employed.  However...

Don't go broke on gear. 

Every DP starting out thinks they'll be more employable if they buy a camera, plus some lights and grip gear, and then some lavs... If you went to film school you'll be in plenty of debt already.  Consider your purchases carefully (see above).  If you want to be a DP, don't become a rental house.  If your dream is to be a gaffer with a 3-ton package, then start with used gear that the rental companies are throwing out and fix it up.  For your own creative work, chances are you don't need the shiniest camera, just one that's good enough.

Others' perception of you is based on your first job with them. 

You may be a budding writer/director, but when you're hired as a costume designer, the producer is always going to see you in that role.  You may get a call from him for your next job, but don't expect him to take your script seriously.  This is not always a given - it depends a lot on the producer - but don't expect to move from one job into another with the same set of people.  Also...

Your boss is also your competitor.

This is not unusual or bad, just something to keep in mind.  If you're an electrician, your gaffer will compete with you in the future for certain jobs.  As a line producer with a few scripts in my pocket, I know that my producers have their own projects they're hoping to show to their bosses (the executive producers).  So they're not super-motivated to move my script past theirs on the pile.  I don't blame them.  This doesn't mean you shouldn't do your job, or that you don't have a lot to learn from your boss.  But don't expect them to advance you past a certain level.  That's your responsibility.  (Your mileage may vary depending on your boss).

This is a business. 

Individuals who work for distribution companies may have fantastic taste and may even believe that their mission is to bring your film to the world's attention.  But distributors are in the business of making money.  If your film can help them do that then they may show you some largess, and even publicize the film.  But don't expect them to be generous or even fair to you, or share your view of your film.

Success is measured in years not months. 

If you're not in it for the long haul, there are easier ways to make a living.  Getting a project from conception to distribution and promotion can take at least five years.  Becoming a union department head can take even longer.  Don't be discouraged.