Most people's first mistake is buying a model train set on a whim, or buying one like dad gave them when they were a kid. Why? Because over and over I hear stories about how they just didn't realize how much space it was all going to take when they got it set up. Many a child has had their delight turn to dismay by the words "you don't have room for more track". So before you buy, know just how much room you have, and buy trains sized appropriately.
I'm a big fan of actually playing with trains, as in changing the track around frequently. This is why I use integrated roadbed track
and work on a felt-covered 4x8 foot table top. Even if you're going to build a permanent layout, I advocate playing with temporary layouts for a while first to get a feel for what you can and can't do. For this purpose a 9x5 foot standard ping pong table is ideal as long as its sturdy. Of course the larger your scale, the sturdier your table needs to be.
Some people use cables and pulleys to lower model railroad tables from the ceiling and raise them for storage. If you don't have experience in building construction/renovation I would recommend using a licensed contractor to build such a thing.
Many people today build modular layouts. Modules frequently vary in length, but usually have a width of 2 feet. Standards are published for track positioning and wiring so that modules built by different people can be connected together. At the 2008 N Scale National Convention a layout of over 500 modules will be set up. Here are links to some module standards sites:
Permanent LayoutsPermanent layouts require serious space planning. Appropriate choice of scale will make a big difference in long term satisfaction or dissatisfaction with having allocated this space. O scale layouts take lots of space. HO and N scale outsell other scales because the average modeler can make space for an HO or N scale layout in their den, basement, or garage. Some apartment dwellers do remarkable things on small tables or shelves with Z scale. In decades past, permanent layouts were frequently built on tables 4x8 feet or larger. The problem with layouts 4x8 feet or is the reach it might require to pick up your derailed trains. A 4x8 is only a problem if it's against the wall, but when space planning always consider your longest reach.
Around the WallsThe trend today is away from large rectangular layouts to narrower layouts against the walls of a room. Today it is much more popular to build two foot wide layouts in an L or a U shape, perhaps with an "island" extending out into the the room making an M. One thing to watch out for when building an around-the-room layout is the reach into the corners remember that if the layout is three feet wide from both sides, the reach to the back of the corner is about over four feet, and you can't walk around to the other side like you can on a free-standing 4x8 foot layout.
Multi-Level LayoutsA variation on the around-the-walls layouts is to put a two-foot shelf above a three or four-foot layout table. In order to move trains between the levels the layout using a helix, a vertical spiral track. Helix construction has become an entire sub-culture in the field of layout construction.
Brush Up On Your Carpentry
If you're planning on constructing a permanent layout complete with trees, hills, cities, and roads, you'll first have to build the benchwork to support it. There are books available that deal entirely with building benchwork for model railroad layouts. Among the most popular is How to Build Model Railroad Benchwork
by Linn Hanson Wescott and Rick Selby.