I wouldn’t completely discount starting university. Online is challenging, but also has some nice benefits to it. Depending on your learning style, ability to manage time, etc. it can be extremely effective.
But as others have suggested, if you’re going to take a full calendar year off, *and* you’re pretty certain about your career path, then spending time developing some practical skills and/or picking up a certification isn’t a bad idea. But what exactly you spend your time on will depend greatly on what you want to do professionally, and you may not have a fully developed idea of what that is yet.
One alternative idea: if you already know what you’ll be majoring in, and you know what school you’ll be going to, take a look at what courses you’ll be taking during your time there. What skills are those classes going to work? Can you spend some time in the coming year working on those skills, so when you’re put to the test, you’ll be better prepared than a student that’s had no exposure to the material.