Well, going on to uni gives you a bit of anonymity, which I found fantastic. You might not like it at first, because you'll go from everyone knowing you and knowing about you, to a lot of people not. And they don't care to know you. I found it very freeing, because it gives you time and space to grow and try new things without the stigma of 'Oh, Toplayusout is doing that?' You wanna go skydiving? Camping? Stay home and read all weekend? Do it. It's your time. Find new hobbies, and take this time to try things that you've wanted to do but didn't have the time for.

You also say that you think you want to be a teacher. That's great, and if you continue along that, I'm sure you'll be a good one. But, if you change your mind through the first year of studies to do something else, that is perfectly fine too. Here in Canada the first year of uni is usually a very softly focused year of study of what you said you'd intend to focus on. It's designed that way so if you get halfway through the year and decide to change, it's easy to do so without wrecking up your whole four year plan. A lot of people change their focus of study, so don't feel bad if you want to. This is the year that you test things out and see what you fit best with.

And if people give you shit for changing it, just tell them that 'yeah, I wanted to be a teacher originally, but then I started focusing on this and I found it a much better fit!' and don't listen to their bullshit. The career designs you make as a younger teen are a good idea, but you can't always get a good grasp on what you'll enjoy studying and working in until you get into it.

That was a ramble, sorry. Hope it helps. TLDR: first year is full of confusion and new things and change. Embrace it and have fun. :)