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You should do this even if you've acted before, because classes will help you hone your skills consistently. Opt for basic acting classes, as well as classes on improvisation and scene study. It's best to join classes intended for young adults. If your school has drama department, join that, too. To find acting classes in your area, start with an internet search. Type in "acting classes + your city" and browse the results. You can also inquire at your local theater about classes. The cost for classes will vary, but they usually fall somewhere between $25 and $100 per class. Group classes are great. They allow you to make theater friends and learn from others. However, if you can afford it, consider also getting a private coach. The one-on-one interaction can really help you grow as an actor. To find an acting coach, start by doing an internet search. Type "private acting coaches + your city" into your favorite browser and check out the results. Inquire about private coaches at your local theater and ask around at your acting class. Private coaches usually cost between $90 and $100 per hour. Act every chance you get! If your school is putting on a play, audition for one of the roles. Check out your local theater company and join their productions. Try out for a variety of different roles so you learn how to be a versatile actor. Help backstage with lighting or props if you aren't able to get an acting role. Look for local audition workshops, musical theater bootcamps, and summer programs to participate in. You can find these activities by doing an online search and asking around at your acting classes. As you gain acting experience, your strengths will start to show themselves. Maybe you're great at dramatic roles, but comedic acting is difficult for you. Continue to hone your dramatic chops, but put more time into getting better at comedy roles. Work on becoming as versatile as possible. Keep in mind that Nickelodeon is a lighthearted channel, so most of the shows will call for comedic acting. Consider working on your singing abilities. Many Nickelodeon auditions will require you to sing an audition song, but it really just depends on the role you're going for.

Summary:
Sign up for weekly acting classes. Get a private acting coach. Get as much acting experience as you can. Figure out your personal strengths as an actor.