You definitely have to pick a major. That’s the degree program you’ll be in. So if you want to study advertising, you’ll be in the advertising (or business) program, and that’s what the majority of your courses will focus on. I don’t know that a minor is required at all universities or in all programs, but @Vincentt‘s description is pretty good.
It all does vary by school, so you’ll have to speak to an academic advisor to get the specific details. At many schools, though, as long as you’re not in a professional program like premed or nursing, you don’t have to declare a major until the end of your second year. I’d suggest you get in touch with someone at the school and talk about it before you sign up for courses. (Or, if you’re in high school, speak to someone in your guidance office – they know all about this kind of stuff.)
Good luck!