There are no guarantees in life, you may have decided where you want to go to university, but they have to agree that they would like you to study there.
As I have mentioned in other posts, I applied a very, very long time ago. So I can in no way say that I am en expert on the selection criteria a university might have. Why not ask them? If you do and they tell you, do me a favour and let me know what they say.
If you have decided to get yourself a design education the most comprehensive way is to go to uni and study graphic design. There are part-time graphic design courses out there, but unless you intend to be a part-time graphic designer (or an evening graphic designer for that matter), the full-time is the way to go.
If you are fresh out of school it may feel like a long time out of your life but trust me, looking back, it is the best time you will ever have. Sure, you won't be the richest person on the planet, but you will have a lot of fun and be as free as a bird.