For a start, in order to become a veterinarian, you need great grades in high school. It takes years of study and lots of memorizing facts so generally, to qualify you will need to be pretty brainy to get a head start. Naturally a love for animals helps - after all, you will be looking after them all your life. There are other things that selectors will look at - for example, previous experience with animals, background situations but there usually is a requirement for good grades in subjects like biology and science.
If you live in America, then to become a veterinarian, you will need to do 4 years of college, and then another 4 years in veterinary school and then pass a veterinary medical board examination before you qualify. There are also requirements for you to take state board examinations if you are planning to practice in another state aside from the one you took your board examination in.
Below are some sites that explain the answers to your questions.
Are you thinking about becoming a Veterinarian?
How to Become a Veterinarian - wikiHow