In my experience, I read a lot of books on famous architects and different styles of architecture (post-modernist, deconstructivist, brutalist, etc), took art and photography classes, went to see examples of famous architecture, loved geometry and physics, etc.
Some high schools and community colleges offer technical drafting and computer modeling classes (the vast majority of firms use AutoCAD or BIM programs like Revit which allow you to generate photorealistic images and animations), so that may be an option for your character.
Some of my classmates did summer internships in high school, so perhaps your character or her parents know a local architect and can set something up. An internship at this age would probably consist of running errands and shadowing the architect on construction sites and zoning meetings and structural / MEP coordination meetings, maybe some physical model-building--absolutely no design. It's very unlikely that the architect will allow the intern to draft anything that's going into construction documents.
Let's see, what else? I started looking for schools pretty early on, because not all universities have accredited architecture programs (at least in the US). The higher ranked the better, of course. There are also two tracks that not all universities offer: one is called a "four plus two" (four year bachelor, either in architecture or not, plus a two year master in architecture), and the other is a five year "professional" bachelor degree (which is the route I took). And school itself is brutal. Lots of all-nighters.
In order to pursue architecture as a career after university, one has to "intern" for approximately three years at a firm. Then, you're eligible to take the ARE exam. After you pass all of the tests, you can then register as an architect.
If it sounds like a lot of work, that's because it is. On our very first day in architecture 101, my professor told the class we should all reconsider. I think my class started with about 120 people, and I graduated with 50.
If you need more info, feel free to PM me. I don't work in architecture any longer, as I left after my three-year internship to get another degree, but I can help you out with any nitty gritty details you may want to know. I'm still in the design industry, just on another side of it.