If you can't write you're in trouble unless you devote time to learn now. You also need to have an appreciation and knowledge of how the law works when it comes to contracts, riders, disputes.
You'll gain that in university with speciality entertainment law courses but for now should take any and all law courses that come up. The bulk of a manager or agent's time is spent on the phone trying to get out of or into lawsuits over copyright infringement, not being paid and allegations an artist plagiarized someone else.
You REALLY, REALLY need to intern at a theatre either this summer or right now in your community. You NEED to know about lighting, audio and other technical things to make sure your artist always sounds right in every venue to write technical riders for venues to set things up correctly.
Any backstage experience in a theatre will help you greatly. You can't represent anyone in entertainment without knowing the process of how live shows are achieved.
A working knowledge of the entertainment industry, trends, music etc. will help you too. Once you reach university there will be programs for Entertainment Management and PR. From there they'll teach you everything you need to know.
For now join student council or anything where you can represent others and be a leader. College recruiters will want to see that you can write as well as lead if they take you into such courses.
You should also see if LIVE NATION has a field office in your city or near it. They promote 90% of all major concerts worldwide. I figure if they have an office near you that they might let you tour it an observe how they set up and promote concerts. It's worth a try.
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