So you've decided to bite the bullet and construct your own model railroad layout, and necessarily, the table the railway seats upon. Many people start by building their first layouts on a table top, and while this is simplicity personified, it does have its limits. The biggest of these drawbacks is that everything must be on top, and nothing can be below ground level (or table level) that's when most modelers realize the need to build their own tables to be able to accommodate any sort of scenery and landscape they could dream up.
First, you need to plan carefully the size, relative weight of different sections, and design of your new layout. If one section is more scenery intensive, it will require more attention as to support. An L-girder support system has become the standard way to support your model layout, and can be adapted to almost any configuration, so providing you with all the strength your layout will need. L-girder bench work is very strong, and still about as inexpensive as you can do. It's a very functional structure, and is easily put together. It is a platform consisting of girders, cross braces and joists, all working together to do the job. Most of the time the joists are about 18 inches apart, but they do not need to be evenly spaced.
Try and use care when selecting the lumber for both the support as well as the actual tabletop. Obviously you'll want flat pieces that will carry your roadbed without problem. Take care in the support pieces as well. Cheaper grades of lumber will warp over time, and I do not have to tell you what that could mean to your layout. If your layout is larger than a standard 4 X 8 piece of plywood, you'll still want to consider having each piece no larger than that for easier transportation you should have to move the layout. Assemble all the joints between the bench work cross members as well as the support risers with wood screws accessible from beneath the layout. This way, you do not have to ruin scenery when looking for some random screw lost in the trees! Make doubly careful not to split the wood in your supports by either using too large of screws or by poor placement of same.
Carefully locate any cuts you want to make for either access or for scenery, and make sure these fit into your overall support scheme. There are varying schools of thought as to whether to saw your holes in the tabletop after the track is in place or before. For me, I think it is better to do it all beforehand, as the very act of sawing, regardless of the quality of your tool, can possibly send enough vibrations through your layout to loosen track and ballast, making your previously a smooth running operation into a clackety-clack mess. Just my 2 cents.
Most tabletop layouts are at about 48 to 55 inches high. Obviously, this is something you can control, and if you've bolted the main support legs in place, (instead of screws or nails) you can even change it at the drop of a wrench! You'll want to try and make it so that even the tops of the highest hills and grades are visible without a step-stool.
Building a killer train table is nothing if not pre-planning. Take some time with yours and you'll be most pleased!