I also want to be an imagineer, so here is my thought process...
(I am sure there are billions of other ways, and I may never end up as an imagineer, but here's just what I am doing

)
The first step for me was my college major, and thus the prep courses in highschool. Imagineers are usually not just people that majored in fun, because Disney only hires the best. And usually that means that, imagineers are engineers with a creative side. So I would try out engineering and see if it interests you.
Next step is getting your foot in the door with Disney. Now the College Program looks like a lot of fun, but it is not as focused on getting you into higher technical positions at the company.
I just finished my first two years in Industrial Engineering and now I am doing a Professional Internship with the Cruise Line. These professional internships put you in the offices and behind the scenes to make sure you are doing what you love. But also getting you trained to see if Disney likes you enough to offer you a full time job when you get out of college. So I HIGHLY recommend this. And this will open all the doors to see if you want to be an imagineer.
But I have realized...being able to work for Disney, no matter if it is picking up trash or being an imagineer, is such an amazing opportunity. It really is the most magical place on earth. So take it one day at a time. Make good grades, love Disney, and hopefully we will work on an Imagineering team together some day! ;-)
Have a magical day!