Have fun now learning and growing as an actor and performer. Take acting classes. Voice lessons and dancing classes can be fun too. Audition for what you can - school plays, community theater, church shows, etc. Join a drama club and compete in forensic speech/drama classes. Join other performance groups like a choir, band, dance troupe, improv group - whatever. Read plays and scripts of all kinds keeping an eye out for characters you love and monologues you want to develop and embrace learning of all kinds.
Look into getting a Bachelor of Fine Arts or Master of Fine Arts degree in Musical Theater. Its not required for a professional career, but it's something you should consider. And if you don't get a degree you do need to get quality training from respected instructors. Where you study and who you study with matters, not only to improve your skills but one way to start to network and make industry connections. And training is not some hoop to jump through. Many actors continue to get training throughout their career.
Since you're interested in a professional career you may want to research to get a realistic understanding of what that involves. It's not going to a bunch of auditions hoping to get a job - it's running a business. That means you have to understand things like agents, unions, contracts, taxes, marketing and networking Scams and rip offs prey on wannabe actors who have some fantasy about being "discovered" and don't understand how the business really works.. Industry websites like Backstage have helpful articles.
With training, experience and a realistic understanding of the industry you can determine if attempting a professional career really is for you.