A lot of people will tell you to get an agent. An agent is only something you should seek once you are a trained, polished, working actor.
You should never pay an agent, agency, manager, or casting director any money or fees prior to you landing a job they sent you on. Then, and only then, do they get paid - and that is a percentage of what you make - usually 10 - 15%.
It seems to be a popular myth in this forum that you NEED an agent to be a working actor. That is simply not true. You can find a lot of auditions and jobs on your own without an agent. I have done professional theatre, films, and national television all without an agent.
If you are really going to go for NYC or LA and have already built up your resume, have years of training, good reputation and press, then, yes, you probably do want to pursue an agent - but an agent is really NOT the place to start.
Everyone here will probably tell you to start at school, taking classes and doing school plays. That is a good way to start. You can also start auditioning for plays and films outside of school. Why not? Know that you are not going to get cast in everything you audition for - but why be content with just school stuff. You learn the most from actually doing.
Without knowing the area in which you live, I cannot give you direct links to auditions in your area. Google is a really handy tool, though, for finding auditions. You can use craig's list to find local auditions - a lot of these are legit - but always use caution and common sense when answering an ad from craig's list. If it sounds too good to be true, it probably is. If they use terminology like "casted" instead of "cast" - they have no idea what they're doing and should be avoided.
ExploreTalent.com is a good tool, if you know how to use it. They charge you to submit for roles you might be good for. But you can look up what films and plays are auditioning in your area - and then use google to look them up and find a direct way (for free) to set up an audition.
You can also use google to look up local teatres. Most theatre sites will tell you how to audition (either with an audition notice on their site) or if it is a more professional theatre, it will tell you where and how they usually hold auditions and/or where to submit your picture and resume.
So... to become an actress...
*Read a lot of plays and familiarize yourself with them. Practice reading them out loud. This will help you for when you audition and you have to read something you've never seen before for the audition.
*Take classes whenever you can. People on here will tell you that no one can teach you how to act. Maybe that's true, I don't know. But what classes will give you is practice. Your teacher will guide you to give a stronger performance and help you polish your craft. You will get to perform in front of other people in a pressure-free environment and you will be challenged to try things you haven't.
*Audition whenever you can. Auditioning is a skill in and of itself. The more you do it, the easier it will become. The hardest thing, especially for an actor starting out, is to not place too much importance on any one audition. If you don't get cast, don't let it crush you - just move on to the next audition. It may be a blessing in disguise! There have been times when I have been really disappointed when I wasn't cast in a show or a role that I thought I really wanted - only to have something much better come along - which I wouldn't have been able to do if I were cast in the other show. The other thing actors tend to do is talk themselves out of going to an audition. Don't do that. GO! Go to all the auditions you can. Don't try to guess what the director is looking for. Don't convince yourself you're not right, not experienced enough, etc. to go. Just go! Challenge yourself. And then feel proud of yourself for going afterwards.
*As you read plays, start collecting monologues that you like. Actors need to have a few monologues on hand and ready in case an audition comes up that requires a monologue instead of just cold reading. Memorize your monologues and work on them. Perform them in front of family, friends - anyone you can get to watch you. If it can be helped, you should never go into an audition with a monologue you have never performed before.
*Once you have some experience (plays, classes, etc.), put together a resume - and keep it up to date. Your resume should have your name in large clear type at the top. Your phone number, e-mail address - for contact information. Never put your address or your social security number on your resume. You should also put your hair color and eye color. Height and Weight is also recommended. If you sing, you should put your voice range as well. These items go toward the top of your resume. Union status would go directly under your name, if you were part of a union.
*Next, your resume will be divided into sections - THEATRE, FILM, TELEVISION, TRAINING, SPECIAL SKILLS. Don't worry about it if you don't have credits in all of these categories - just use the ones you need. For THEATRE - you would create 3 columns - Play - Role - Theatre -
Twelfth Night - Viola - City Shakespeare Company
Mikado - Yum Yum - City G&S Society
Cinderella - Cinderella - City Children's Theatre
The film and television categories would work the same way, except instead of listing a theatre, you would list the production company.
*The TRAINING section of your resume - should include anything you've had training in, which could relate to the theatre and film industry. Acting classes, voice lessons, dance, gymnastics, stage combat, teleprompter, acting for film, etc. These should be listed - by Skill - Teacher - School or Studio
*The SPECIAL SKILLS section of your resume should include anything you do well, that you think could come in handy - accents, musical instruments you play, juggling, riding a unicycle, any sports you can play, etc. For beginning actors - training and special skills are probably going to be the biggest categories on your resume, but don't worry - that will change.
*Eventually you will need a headshot. As an actress just starting out, this shouldn't really be your first priority. However, if you are so inclined, you can easily google headshot photographers in your area. Or even have a friend with a really good digital camera take some pictures of you with a blank background. Take a look at some professional headshots first, so you can get an idea of the look.
*Headshots are 8x10 - and your resume gets stapled to the back of it - which means you need to make sure you cut your resume down to size.
*There are always auditions - no matter where you live. Some people think that just because you don't live in NYC or LA you can't work as an actor. There are theatres everywhere. There are people making independent films everywhere. There are people shooting industrials and training films everywhere. You just have to seek them out.
*As an actor, you have to be resourceful and smart. Don't fall for scams - any place claiming that they can make you a star and charges a lot of money. It doesn't work that way. Acting schools that charge thousands of dollars (other than legit colleges and academies.) Just be cautious and use your common sense. If something doesn't feel right - go with your gut and avoid it. There will ALWAYS be other opportunities. Check the arts section of your local newspapers - often auditions are listed there. Use google to find auditions. Don't sign up for a site that charges money. Most are scammy - or like Backstage - you're not ready for.