If you are still in high school, take ALL the science classes available. Focus primarily on biology, of course, but you will need the chemistry and physics for university. Learn another language besides English. Try to get internships at vet clinics where you live and become connected to your DVM. That person will be VERY IMPORTANT in helping you get into vet schools.
Veterinarian colleges are more difficult to get into than medical schools. You must have very excellent grades across the board, do well on SAT and ACT and have some strong recommendations in your portfolio. If you decide to work on large animals, go work on a cattle ranch. It is always better to have practical experience in your resume'. If you are going for small animals, the vet clinic work will be important. Actually, you should try to do both work experiences. Take your time. Learn that animals can't tell you what's wrong with them. You have to be a very strong diagnostician in everything you do.
I love vets. I have had cats forever and my vet is part of my family in that sense.