Please continue to read and to generate more specific questions before you open your checkbook, unless it is to pay for some good reading material.
One of the great bibles of model railroading is the late John Armstrong's Track Planning for Realistic Operation. You can get it, and other excellent how-to books from Kalmbach Publishing, Inc.
There is no reason not to go ahead and perhaps think about purchasing a good engine or two, and some quality cars to match, but ideally you would have a firm notion about the 'era' you wish to model or to represent on your train system...what we call a 'layout'. If you really prefer modern diesels, then the structures and paraphernalia associated with the early steam and the transition era between steam and diesel won't have any appeal to you.
If you are feeling a deep burn to start buying, be careful. For the most part, train sets are often not great investments. Bachmann Spectrum used to have some good sets, and Atlas must as well. Stay away from simple Bachmann sets or older Life Like ones. The best engines are from Bachmann Spectrum, Athearn Genesis, Atlas, Broadway Limited Imports, and Proto 2000 from Walthers...in no particular order. The same lines of rolling stock are also good. But have a good idea what you want in a railroad before you open your wallet. It gets expensive moving too soon.
One of the keys in this hobby, I can't stress enough, is self-education. Simple track plans quickly become boring and embarrassing. That is why I urge you to study a bit about real railroad operations and to try to mimick some of that in your small space. That feature is what keeps adults coming back to their creations and to the fun and variety they offer.
-Crandell
