First off, Lionel Polar Express, you might want to check out the following link and then talk it over with your parents to see how much space you can have...
Scroll down on the page and you will see links for all sorts of small layout designs. You'll want to check out all of these. Lionel's traditional 027 track will offer the most variety for the smallest spaces and is the most affordable track system out there, new or used. You can build a decent layout on a hollow core door with 027 track.
There's also some videos on YouTube of layouts built on a hollow core door, if you search for them.
FasTrack, which now comes in all Lionel starter sets, doesn't have the small space flexibility, but offers other advantages. I'm not sure, but the Lionel Polar Express Berkshire might not clear the switch housing on 027 switches. So there's an advantage of FasTrack.
I'd second the recommendation of the LCCA Open House if you can get to the event. But you can still find older Lionel sets, still sealed, or lightly used that came with 027 track.
Your small space (and probably small budget) will limit the types of trains you can run, but these types of trains are usually the most affordable. The Lionel 4-4-2, 2-4-2 and 0-8-0 steam sets are good bets for small spaces. There's a plenty of older MPC and postwar Lionel engines that run on tight curves and look good doing so.
I suppose the first question you need to answer is how much space you will be allowed. That will help determine what type of track you'll use and what sorts of trains you will be looking for.
