So you have decided to bite the bullet and start your model railroad? Good work! That's a great idea indeed. One of the first decisions you need to make is how much space do you have and what type of table are you going to use to lay it on. The most obvious choice is a table top that you have available or a sheet of plywood. This does not always work well though as it has restrictions.
The best option and of course the most expensive is to get a table designed just for your mode train set. These are actually sold online and in hobby shops if you look around for them. These tables are designed with the right sort of curves and gradients that work so well for model trains.
Make sure if you are doing it yourself, that the surface you choose, which is probably going to be wood is nice and flat with no dents or bits sticking up or your train will continue to be derailed. You also need to make sure the table you use is going to be strong enough to hold the weight of your train and the scenery. This really does depend on the scale of model train that you use.
I personally believe that you should layout your scenery first before laying out the track. This way you can cut out any bits of table that you need to, to fit the scenery. This way the vibration from any cutting or sawing will not disturb the track.
Standard model train tabletops around around 55 inches high. This of course can be adjusted if you have bolted it down instead of nailed it down or even glued. it. This way you don't need to stand on anything to have to work on your layout and scenery.