As a freshman, i would say the best way to prep for the DAT would be, if possible, gear your courses towards the material that will be covered on the DAT, this makes studying for the DAT much less demanding as you will inherently be studying for the DAT while you study for your classes. Its a little trick i learned this year as a sohphomore taking it it 16 hours...advanced courses that go more in depth than intro bio like human physiology, genetics, immunology, ochem, and gen chem, (the latter two you are going to take anyway for prereqs) will definately help you in the nat sci portion of the exam. as for when you get closer, i'd suggest practice exams, lots and lots of practice exams. by taking practice exams you can get a feel for what you know and what you dont know and spend more focus on the areas you're not satisfied with. but dont fret about it too much, its only your freshman year so unless you plan on graduating early you'll have plenty of time.