Confidence n. A feeling of self-assurance arising from one’s appreciation of ones own abilities or qualities.
Performing of any kind starts with confidence—the feeling that no matter what happens you’ll do well; that the work you do will be worth watching. In essense it is the actor’s degree of certainty that the work will be successful, established through a history of successes. Each little success in class, in the audition or on set helps build the body of experience that (a) makes for a professional quality performance and (b) comes across on camera as confidence.
To establish a history of success requires dedicated practice. The working environment in film, television and commercials is always changing, so actors have to practice a hundred ways, in a hundred different situations, a hundred times. An aspiring professional actor should put at least ten hours a week into their craft: on stage, in front of the camera, taking classes, reading scripts+plays, dancing, singing, voice training, going to the gym, etc. That’s the professional work ethic.
Good habits help you make the most of your practice. The more your practice habits match the needs of the working environment, the more each success will prepare you for your next audition, or for your next day on set. So learn as much as you can about auditions and about being on set. Read extensively. Take classes taught by working actors. Get a coach. Get an agent. Audition for everything. Do student films. Do theatre. Put yourself out there. And practice, practice, practice.