Ok Nick (?),
I see you are just a couple of weeks older than one of my sons, so I have a pretty good feeling for your enthusiasm! First don't lose that. Second, you need to breathe in and out slowly a few times, because as we all have learned this a a hobby for the long haul. But I think there are challenges, fun, and opportunities to learn in all phases, so it is all good!
So, you've got your space negotiated. That's a good thing. But the last thing you want to do is rush to fill it, or you'll get well started and realize you don't have anything like what you want.
First, I think you need to get and read John Armstrong's book "Track Planning for Realistic Operation". In it he talks about givens and druthers. Once you actually think about it and write them down a lot of other things start to fall into place. I'd also think about getting one or more of Armstrong's layout compilations (I don't remember the titles, look at Kalmbach's site and you should see several). In these books he has the list of givens and druthers from his client (he designed layouts professionally), and then the trackplan he designed, and the thought process that drove the design. It really helps to see how a design evolves. When you see those designs you will almost certailly find one you think is great, but remember, the people building these were not beginners. The ideas are great, but starting off too big could be the kiss of death.
After you do that you'll find yourself doodling a lot. Just don't get caught at school! Eventually you'll get a picture in your head and you will want to get it on paper, and you'll realize that you are not a draftsman (if you are I apologize!). So you will want help. That's where Xtrkcad comes in. Go to
http://www.sillub.com/ and download it. There is a link in the middle of the page that tells how to register for free. Go through the demo/tutorial under the help menu, or you will be lost. Then you can doodle on the computer.
I would not try to fill the garage at first. I think maybe a U shape you can walk into that is 10x15 or so might be a good start. If you are clever you might think about how it could be added on to without starting over.
If you'd like something else to read, here's a link to a good site for thinking about yard design:
http://www.housatonicrr.com/yard_des.html .
If you can answer the givens and druthers questions there are people here who will be happy to help scheme out a design, they just need a bit more to go on. I know that you might not have a era in mind, but in the long run you probably want one. Late steam, with the Challenger, although you could go modern and run it on excursions. Modern has the advantage of being right outside your door.
Enough rambling from me, I hope this gives you some things to think about and get started!
Jeff