Volunteer at a shelter or animal rescue group. Volunteering allows you to get an idea of what caring for sick animals is really like before you make your decision. It also allows you to show you have experience caring for animals when it comes time to apply for veterinary school -- many veterinary schools require some form of experience in working with animals.
Get a job working for a veterinary clinic. Clinics hire employees to do tasks such as sterilizing equipment, cleaning cages and feeding patients. A recommendation from a practicing veterinarian and work experience in the field are powerful advantages over other applicants.
Get good grades in school. Since there are so many students competing for a limited number of openings, veterinary colleges can take their pick. While you don't necessarily need to be a straight A+ student, the higher your grades are, the better your chances.
Take your preliminary courses. It takes three years of regular college course work to apply for vet school, but most students get a four-year degree before moving on. Students should consider a degree that includes classes in sciences and math. Students with four-year degrees are favored -- many students don't get accepted to vet school until after they graduate from undergraduate school.
Decide with what type of animals you want to work. Veterinarians generally fall into three categories. Large animal vets work with livestock such as horses, cattle and sheep. Small animal vets work with companion animals like dogs and cats. Exotic animal vets work with wildlife and exotic pets -- everything from birds to snakes. The type of animals with which you want to work will determine to which schools you should apply.
Visit the websites of the veterinary schools in which you are interested. They will have information on how to apply and what their specific requirements are. You can talk to counselors, and even visit the school.