First, I'd recommend joining a sales team. Learn the ins and outs of being a sales person, and decide what industry and sales type you'd like to manage (inside sales, outside sales, inbound call center). Experience is the best teacher, in my opinion, and having sales experience will give you the best insight into how to be a good sales manager.
To be a good manager, would like to suggest having management knowledge plus practice. It is important you have good communication skillset, and understand people from different angle which includes culture background, gender, age etc. All these experience come from practice. Reading book can give your knowledge, real practice can provide chance to be a successful manager.
1) During your school holidays, do a temporary job in Sales. It can be selling any product. It gives you the exposure of working as a Sales person, talking to potential customers, pitching for the products or services, and understand the entire sales process.
2) Join a sales organization after you graduate. You may have to start from the bottom. Some large organizations have trainee programs, and you may start as Sales support role.
3) Take online courses on Sales to develop your skills. Eg. Presentation skills, negotiation skills, etc.
All the best!
If management is your calling you must scale your leadership capacity.
Read a book or listen to a book on leadership, sales, or self development.
Its unfortunate that they don’t teach these skills to often in a school setting.
Studies show that if you study any give subject for 5 minutes a day, within 2 years you would gain experience worth a college degree.
Some of my faves would be, Leadership and Self Deception , The One Minute Manger, and The 7 Habits Of Highly Effective People.
Also... YouTube Leadership, Sales, or Motivational Speakers.
Listening to great leaders will help you adopt their attributes as a leader and speaker.
If leadership is your calling start now!