Start taking the pre-requisites that will help you out on the DAT..
1st semester: Gen Chem & Bio I w/ labs
2nd: Gen Chem & Bio II w/ labs
3rd: OChem I w/ lab
4th: Ochem II w/ lab
Generally take the DAT the summer after junior year, but you will technically have taken all the required classes by the end of your sophomore year, so its up to you whether you choose to get it out of the way or not. Personally, I think some of my upper level bio classes like physiology, genetics, plant bio, etc helped me out, so you may want to hold off until after you take some of those classes.
And of course like Tedy said, show your interest in dentistry through job shadowing. Get involved with the pre-dental society at your school (if you have one). Get involved with extracurriculars and volunteer activities that will strengthen your application from the get-go. Good luck!