What do I need to do to ensure I can become a veterinarian?

If you are in high school you should take as much science and math as possible, advanced placement or college level courses if possible. Talk to the counselors or a trusted teacher at your school about how to prepare & apply.

You should also contact the Office of Admissions CVM UIUC :

https://vetmed.illinois.edu/education/doctor-veterinary-medicine-degree/accreditation-information-for-applicants-and-students/

https://vetmed.illinois.edu/education/doctor-veterinary-medicine-degree/admissions/

Part of their job is to help applicants prepare.

Visit with local veterinarians, hopefully some that are U of I alumni, and ask them for advice. Any experience you can get with animals, especially horses will be helpful.

One other thing you should prepare for, and that is if you don't get accepted to Vet School on the first application. I got in on my second application, and worked in the year in between. It's always good to have a plan B. Spend at least a little time thinking about what you would like to do if Vet school doesn't work out. Having said that, Good Luck, keep trying & don't give up when you hit a set back.