I appreciate that you are choosing Dentistry as your career path. To answer your question, you do not have to be a dental hygienist in order to apply for the Dentist. I strongly recommend shadowing in Dental office to strength your resume and to know whether dentistry is the one you want it.
I hope it answers your question.
once you've found your thing, ask a ton of questions while you're shadowing. this is how your instructors will know you're serious about it. go online & read articles, then go to your mentor & ask an intelligent question. they'll be impressed. they will know that you're just a kid and won't expect you to know anything. they will, however, be pushing to see if you can read something & try to figure out more in your head. the articles you read will matter later (read the new england journal of medicine when it makes sense to you, for example); something that sounds like science in the national enquirer won't impress anyone. maybe a speech by NDT is a good place to start, or even billnyethescienceguy. find a sentence that's just a little bit over your head, look it up online to try to understand it and then ask your mentor to explain it. that's 'impressing your mentor 101'. it's not about getting all the right answers. it's about learning how to think, how to learn, form a complex sentence, all those professor-y skills.