Here is the first and most important question to ask yourself, DO YOU WANT TO ACT or DO YOU WANT TO BE FAMOUS? There is nobody who can answer this question but you, and there is no reason to lie to yourself. There is nothing wrong with wanting to succeed and even wanting fame and/or money. But if it is your ONLY reason for wanting to act, stop now. You will not succeed, and if you do succeed, it will feel empty and dissapointing. Find what you are passionate about in life and in performing, is is singing, dancing, acting? Find it, and start from there.
That was a tough question to ask yourself, but know this...if you truly want to act, you absolutely can and will for the rest of your life. Honestly. There is work to be had everywhere, local theater, local commercials, singing telegrams, improv comedy groups, anything. If you want to act, you will!
Next, ask yourself if you are talented. We have all seen the painfully bad auditions on "American Idol" and "America's Got Talent", as well as other shows. Don't you always wonder to yourself, "Why didn't anyone ever tell them they're terrible?" Well, start by being honest with yourself. Take care of yourself and work hard, don't look to others to make this happen for you. Don't compare yourself to others, or you'll never feel good about what you are trying to do, but do honestly asess your own gifts, skills, and talents. What are you inclined to do? What are the industry standards where you are headed? What kind of work do you want to do, and how do you compare to others already working there? Be honest with yourself, and also gentle. Be forgiving and willing to work.
So, you've asked yourself why you want to act, and you've asked yourself where you are right now. Now it's time to work! Even if you are remarkably talented with all the potential in the world, you have room to improve. Find a class, a group acting class. Or a coach, or a more specific class like "acting for commericals". Take a dance class or a voice lesson, do anything you can to learn. And don't be hard on yourself, this is a life-long process that will never stop. Nobody is perfect, so go in willing to work.
Audition. Audition, audition, audition. Even if you haven't taken a single class yet, and you think you aren't ready, go find an audition! Too many actors and actresses say they are, "waiting until they are ready", and the truth is you'll never be prepared until you know what it's like to audition. Of course, be prepared to get the part. If you can't fulfill the commitment to the job, don't audition. BUT, auditions are the best learning experiences you will ever go through. They will teach you how to handle criticism and what kind of standard you have to be up to in order to get the jobs you want.
Know how to take criticism. DO NOT EVER TAKE IT PERSONAL, good or bad. Some people will give you helpful criticism that you should take and use, like, "You need to work on being more believable," or, "You need to act more with your face and less with your hands." and so on and so forth. Others will say things like, "You are too fat." or "You look too old." These are the people to IGNORE. Their criticism doesn't mean a thing. Anyone mean-spirited or inconsiderate isn't trying to help make you a better actress. They are only bitter, probably because they are miserable. Don't even waste time being mad at them. Just move on. Look at the different types of people in film, theater, and television. There are roles for old, young, large, small, tall, short, etc. So don't try to be something you're not. Just stay healthy and move forward!
Always have material prepared to show your skills. Typically, you should always have a dramatic and comedic monologue ready, each about a minute long. DO NOT pick monologues with accents, and DO NOT explain what is going to happen in your monologue. Keep is simple, all about the acting.
You should also have 16 bars of a fast song, and 16 bars of a slow song ready to sing, with sheet music in your key OR, be ready to sing them a capella if there is no accompanist. Bottom line, BE READY at all times to show your skills.
Be confident. Not arrogant. Directors and casting directors want to hire people who are pleasant to work with and have around, so be yourself and be calm in an audition. And ALWAYS be on time. In fact, be early. An audition is just a different form of a job interview and should be treated as such, with professionalism. No cell phone, no gum chewing, and dress conservatively, unless otherwise instructed to dress like your character. I've often seen people told to take off an type of costuming before an audition, it can be distracting. So wear simple but dressy street clothes.
Stay focused. There are many distractions along the way to becoming/or while being an actress. People will tell you that you need to be "on the scene" or something like that. Or that you need to be seen to open doors. While it's true that you need to form relationships, those relastionships will come naturally out of auditioning, time and time again as directors and casting directors see you over and over. They will NOT come out of hanging out in bars or restaurants, like some people will try to tell you.