Commit yourself to studying your trade. Enroll in classes or attend a local acting school. Find a teacher or mentor you connect with on an emotional and intellectual level who has a proven track record of success. Keep an open mind, listen and learn.
Watch a variety of movies. Start with the classics, and work your way up to current movies. Find actors you identify with and who fit your style, and then study the voice intonations, facial expressions, body movements and timing of the actor.
Listen to others. Pay attention to what more experienced actors are saying when they offer advice, encouragement, observations and even (especially) criticisms. On stage, listen to your scene partners to hear what they are saying. Much of acting is reacting to other characters, and listening to them makes your performance more believable.
Strive to shrug off negative comments, rejections and failed auditions. Attempt to keep centered, and refuse to allow yourself to lose focus on why you wanted to become an actor in the first place.
Concentrate on your facial expressions. Convincing acting depends heavily on what the audience sees in your face. Practice combining the proper facial expression with the vocal tone you are using in front of a mirror. For example, exclaim "Oh!" with a look of surprise, and then work on an entire array of emotions, such as anger, frustration, happiness, elation, pensiveness, shyness and awe.
Practice your lines until you really know them. Practice them in the morning, noon and evening. Practice while driving, in the shower, when taking a walk or working out, when you wake up and before you fall asleep at night. When you know your lines, your confidence increases dramatically and frees your mind to concentrate on the more creative aspects of portraying your character.
Learn to relax. Great actors constantly work on learning how to relax the muscles in their faces and bodies at will. Tension is obvious when on stage or in front of a camera; your body seems stiff and jerky, and your voice becomes tight and higher than normal. Strive to relax using any technique that works for you.