What undergrad are you going to go to? U of Okla? I'm sure their dental program is good (most state schools are).
I can't comment much on your dental assisting program. I shadowed dentists, but unfortunately never got any hands on experience. Sounds like a great idea though. I'm sure working as an assistant would be a big advantage when applying to dental school. One question: are you going from HS directly into college and just work as an assistant in your spare time?
Once you're in college the most important thing to keep in mind are dent school requirements. Usually this includes: 2 sememsters of general chem, organic chem and physics; 1 or 2 semesters of general biology; 1 or 2 semester of English; and maybe a semester of calculus, biochem, upper level bio (genetics, micro, etc) or some other other odd class. Best to check with U of Okla or whereever else you may want to apply.
Now that you have requirements in mind, choose a major. A common question is whether dental schools prefer one major over another. No. You can major in whatever you want as long as you take the requirements. In fact, IMO dental schools like to see well rounder students so majoring in something other than bio or biochem may be advantageous. Major in what interests you and what you think you'll do well in. GPA is the name of the game. FYI, I majored in biochem. I knew I could get more A's in science than humanities, and biochem interests me.
Specific courses? Besides the requirements you will also need Physiology (IMO) for the DAT and dental school prep. Other classes helpful are histology (cells & tissues), microbiology, and biochemistry, but this advice you won't need for another 2-3 yrs. Remember to take classes that interest you. Art, history, literature, poli sci etc.
Other things to consider are community service, organizations and leadership, and research. For community service there are tons or organizations on campus devoted to this (Alpha Phi Omega) not to mention the United Way, Red Cross, or your church. Join an organizations that you enjoy. Look for leadership opportunity. All these things help. If you're interested in research ask your advisors or check with the advisors of the bio/biochem dept. If all else fails email around. For your first year it's hard to find a research job b/c you don't know anything. Often you can start as a dishwasher and work your way up.
Good luck