Hello!
In order to have a competitive application, it is recommended to have the following:
-Around a 3.5 GPA.
-A competitive DAT score. I'd say a round 19 is pretty good, others argue that it should be higher.
-At least some shadowing. Some schools require a certain number of hours, look into this.
Also, when it comes to the application itself(which, if you want continuous schooling, you should apply in the summer after junior year), you will need to write a personal statement(PS) as to why you want to be a dentist. SDN has a PS reading thread that you can send to one of the members listed and they'll help you with grammar and whatnot.
Save some money between then and now. Application cycles are expensive. Use the ADEA book and make a school list. Most recommend that you should apply to 10-15 schools.
This is sort of an overview, but if you have a specific question then I'll try to help out if I can. =D
PS: If you decided to take a gap year, you'd be in my dental class. xD