Just like there are all different types of personalities that become doctors or lawyers, there are all different personality types that become actors. And you can always have acting as part of your life, even if you choose not to pursue it as a professional career.
You can't jump start your "career" mainly because what is expected from a 14-year-old (and older) is VERY different than what is expected from a 4-6 year old. Practicing monologues in a mirror is not the best way to practice acting. Acting involves interacting and reacting with others, so you need to get out an actually act. Take some acting classes. Voice and dance lessons can help too. Audition for what you can - school plays, community theater, church shows, whatever. Some other things you can do:
* Join (or start) a drama club and look into competing in the acting divisions of forensic speech/drama competitions.
* Join other performing groups - choir, band, dance troupe or the like.
* Read plays and scripts of all kinds and find characters you love and monologue you want to practice and perform for friends/family.
* Volunteer to read out loud to groups or record books on tape to practice your voice acting skills. And not just cold readings - but where you study the text and make good decisions about how to bring the character and story to life.
* Contact local film schools and ask how they find actors for student films. This is a long shot since they're mainly use adults, but you can try.
* Get together with friends and make your own videos for fun.
* Start an improv group at school. Look online for improv games you could do and at first just get together and practice them. Then if you get better, you can look into actually performing for people.
* Read books and websites about the craft of acting and the business end of acting. "Acting for Young Actors: The Ultimate Teen Guide Paperback" by Mary Lou Belli and Dinah Lenney might be good for you. "
Basically have fun learning and growing as an actor. Then when you get older you can do more research and learn about what is involved in pursuing a professional acting career and check out some acting programs for after high school.
And yes, shy people can be actors. But like everyone else they should pursue an acting career because they love acting - not because they want to be famous or popular or because they want to escape their boring life or things like that. Being an actor doesn't make problems go away - you will still be who you are. Rather than hating that you're reserved - embrace who you are and celebrate it. Find how to use it as a strength.
Like I said, you can always have acting as part of your life. If you think about pursing a professional career, understand that is a business. It's like running a company and you are the product that has to be marketed and sold. So learn the realities of the industry and make sure it's something you really want to do.