Be prepared for a series of assistant-type roles and , at the start be prepared to work as an intern, basically doing whatever needs to be done. Must check ego and sense of entitlement at the door. If you have training in a specific sector of the business (eg- camera dept., sound, lighting....) but little professional experience, be prepared to start at the bottom and gradually learn and work your way in, and up the ladder. Your hard work and personal qualities will earn you respect and that enables upward mobility, learning new skills, and opportunities to move ibto higher and more demanding (and better paying) positions.
The industry, in any sector, requires hard work, humility (especially in the beginning) and punctuality. Observe and take note of everything. Its normal to make mistakes at the start, but hard work and a great attitude allow you to keep working and improving your position.
Now that I've said so much about humility and hard work, don't let anyone take serious advantage of you, coax you into anything dangerous, etc..
Despite the proliferation of film schools, the industry is still very much of an apprentice-driven one. With that in mind, take any funds you might be thinking of spending on film school and use them to live on while apprenticing yourself to film productions. That way, you get to experience all of the crafts in a collaborative environment.
Have a thick skin. On low budget productions, there are pressures associated with people having to do too much too fast with not enough resources to do them. In big budget film, someone spent a lot of money and expects a high quality return for their investment. During long hours, tempers get short. Don't take things too personally and put the work ahead of personal squabbles and differences. What is on the screen will live far longer than what happened behind the scenes.