The first thing you need to consider when you want to work as an actor or an actress is that you need to be willing to go to a lot of auditions. Accept the fact that you are going to be getting a lot of rejections at first. There are many, many people who want even bit parts in commercials, television and movies and the competition can be fairly fierce. When you want to make sure you get the work you are after, go to every audition you possibly can. The more auditions you go to, the better your chances are of getting hired. Remember even an audition that doesn't pan out may let you make an impression on a casting director and he or she will remember you next time!
Though you are not going to need an agent, you will need to join a casting service. As long as you live in a major city, a casting service can help get you into the auditions that you are interested in. You do not need an agent to get you into a casting service. Signing up for these services is free of charge if you want to submit yourself audition by audition, but with a yearly fee, you can submit yourself for all the auditions they are providing. On the other hand, if you are just getting started or if you are simply curious, you might like to submit yourself as an extra.
Do not forget how important good pictures are going to be to your career If you want to become an actor or an actress, remember that submitting photos is going to be a big part of that. You will need to pay to submit more than two headshots, but in many cases, it can be worth it, especially if you have more than one "look" that you feel can be marketed. Remember that your photos should always be clear, professional looking and present you in the best light. If you can pass for several different types, remember this is a strength that can get you considered for more film and actor jobs!
If you want to act, remember that you do not always need an agent to do it; see how far you can get just on your own!