When I was at college, I applied for vet school, and got accepted, but later turned my place down in favour of another degree. Most vet schools are 5 year courses, the exceptions to this are Cambridge and London, who do 6 year courses. You will need A-levels in Biology, Chemistry and either Maths or Physics, although preferably both, and you will need to have achieved A grades in all subjects. In addition you also "need" experience working with at least five different species of animals, eg, sheep, cows, horses, dogs, birds. I use the term need loosely, as although they do not say this on their criteria, I can assure you it is a big part of the selection process - the more animals you have worked with the better. You also need to have spend at least 4 months worth of time in veterinary practises (this doesn't need to be a four month stint, I spent several weeks of each summer and easter holidays at local practises). The more practises you can get to the better. Start now, write letters to local vets. Some wont let you observe operations until you are over 16, but most will let you at least sit in on consultations etc.
Also look up the vetsix conferences - these are very good to attend, a lot of influential members of interview panels take lectures over the three days, and it is good to get your face seen. I am not sure what the age limit on these is, I think you have to be over 16, but it is worth having a look at them just incase.
When it comes to interviews, you will need quite in depth biological and chemistry knowledge, as well as recommendations from vets that you have worked with. But that is a long way off yet. If you want any coaching let me know, as I have been there, done it and got the T-shirt.
As someone else has already said, if you are unsure it would be good to aim for a vet degree, that way if you change your mind, you will still be able to go for an ag degree. If you do it the other way round, you will not have sufficient grounding to allow you to apply for a vet degree.
Another option, which has now become common place is to complete one degree, and them apply to vet college as a second degree. This results in a four year course to achieve a vet degree (rather than five years) as years one and two are lumped together in one year. Of course, this would mean a total of 7 years studying, although you don't have to go straight into a vet degree - you can always go back to it later in life.