There are a variety of paths to becoming an event planner and I'd encourage you to do as much volunteering on events as you can, and also to get a variety of internships. Volunteering on events at your school, school clubs/organizations, local non-profits, your church/community can give you a good view into how events are produced. However, it would be pretty rare for you to get volunteer experience on events that are produced by an in-house company event planner. For that, I'd recommend getting an internship with a large company where you can specifically intern with their event planner. And set up loads of informational interviews with folks who work at event planning firms, folks who plan events in-house for companies, and folks on the hospitality side (like hotels, caterers and DMCs). By volunteering, interning, and listening to experienced planners, you may be able to get a picture of which path sounds right for you.