Select a ScaleFirst you need to select a scale. The scale of a model train is its size compared to the size of a real train. The most logical choices for beginners are O, HO, N, and Z scale. If you live in the UK, add OO scale to that list. These are listed in order of their size, largest to smallest. I can give you diagrams that show relative size on a web page, but you have to actually look at model trains to fully grasp he actual sizes. Visit a local hobby store that carries model trains and look at trains of the various scales close-up. HO is the largest selling scale worldwide, with N scale in second place.
Home is Where You Hang Your Hat... and Lay Your TrackIf you are planning on constructing a complete layout with landscape, cities, and roads then you need to plan space in your home for it. Some people build layouts in O scale, but this takes a lot of room. HO and N outsell other scales because the average modeler can make space for an HO or N layout in their den, basement, or garage. Apartment dwellers do some remarkable things with Z scale. If you're not planning on building a permanent layout, then a 5' x 9' ping-pong table is a great place to put up your tracks and run your trains.
Pick a Railroad... Any RailroadThere have been literally hundreds of railroads since the invention of the steam locomotive. Assets have become consolidated in the railroad industry, just as in so many other industries, and there are fewer railroads today. Among the survivors are the Union Pacific, Burlington Northern & Santa Fe, and the Canadian National. But lots of modelers still choose to model a "fallen flag", a railroad that has been taken over by a larger company. Many model railroaders choose to model the railroad whose trains they remember seeing most as a child.
Plan Your LayoutOnce you know your scale and available space, you can start planning your layout. There are a number of track planning software packages to help you. But beginners will benefit from looking at some layout plans posted by model railroading manufacturers. Bear in mind that these plans specify the use of the manufacturer's own track products.
Choose Your Brand... of TrackIf you buy a train "set" it usually comes with an oval of track. If you're serious about pursuing model railroading as a hobby, and you've planned the space for your layout, then you're going to outgrow the oval very quickly. When children are given model railroad sets as gifts, the first thing they usually want is more track. Additional track isn't too expensive; its the turnouts... the "Y" shaped pieces that connect to different rail lines that will consume most of your track budget. Some manufacturers offer expansion sets. But be careful, one brand of track may not be compatible with another brand.
Power to the TrainsA packaged train set will usually come with a transformer or power pack that connects to the rails. However, in recent years more and more serious model railroaders are turning to Digital Command Control, a new industry standard for powering and controlling the trains using computer technology. Some high-end sets no longer include a power pack on the idea that the buyer may want to have the locomotive outfitted with a DCC "decoder" and buy a DCC system to run it with. If you are serious about the hobby I recommend you start out with DCC, or at lest make sure that the locomotives you buy are "DCC Ready".
There's a Time and a Place for Everything
So where exactly is your railroad? The Canadian Rockies? The plains of Texas? Downtown Chicago?
What time of year is it? Is it Spring, with green grass and colorful flowers? Is it Fall with trees in reds, browns, and golds? Or is a blanket of mid-Winter snow covering everything?
What year is it? 1888, 1952, or 2002?.
You need to decide these things before investing in any trains, landscaping materials, or structure kits. You don't want to be running a 21st Century Shinkansen Bullet Train along side an 1880s Consolidation steamer... or maybe you do.