Space and Scale
Before you even think of building a model train set, you need to know just how much space youll have for it. When you purchase a model train, it has a scale associated with it. The scale informs you of the size of the train. The majority of model train enthusiasts love the HO scale because it takes limited space for a regular setup and the detail is nice. If space is at a premium for you, look into N scale trains. These can be run in the smallest of spaces. One of my favorites is the O Scale, sometimes called O Gauge, trains. These are big boys and come with great detail. If you have at least a 6 foot by 6 foot space, you can setup a decent O scale layout. O scale model trains and accessories are typically more expensive, but they are worth it in my opinion.
Requirements
It is possible to setup your own model railroad piece by piece or to buy a complete kit. Generally, a complete kit will lack the frills associated with model railroad setups like scenery, but you can be up and running in minutes. If you end up buying a ready-to-run complete set, you can add-on to it easily by simply buying the parts you desire. Also you can run other trains on the track. Just make sure to buy the same scale as your setup. For example, if you bought a HO scale set, you want to buy HO model trains in the future.
Some Things to Look For
When buying your new model train, look to see if it is DCC enabled. This is a big one. You want to have your train DCC enabled, if possible, so you can run other trains at the same time with ease. Should you decide one train is enough for you, this is less of an issue, in my opinion. However, you never know. Your model train store will only have a very small fraction of the model trains and accessories that are out there. Dont feel like you have to buy what they have. The Internet is full of model train stores.
Finally, I recommend that you look for a model train store in your area and go talk to them. You can learn a lot very quickly by talking with someone in person who cares about the hobby. While hobby stores are ok, a store dedicated to model railroading is far superior in every aspect. The probability of finding someone who really knows about model railroading in detail at a general hobby store is minimal at best.