Honestly, the best thing you can do at 16 is just start building something real. Run a small Instagram page, help a local business with their social media-anything where you can point to actual results later. Coursework and grades matter, sure, but hiring managers light up when they see "I grew this from zero to 5,000 followers" far more than a perfect GPA.
Try to land a volunteer gig or a small paid role managing content for a nearby shop or charity before you graduate. Even unpaid work gives you a portfolio, and that's what opens doors for internships in your first year of college. Most marketing students walk in with zero real-world experience, and you can absolutely leave them in the dust just by starting now. The big picture is all about taking that first step-and you're already thinking about it, which is half the battle won.