Building my structure

Google and find a copy of 'HOW TO BUILD MODEL RAILROAD BENCHWORK"  Read that and decide if you have the skills and tools to build the bench work.  Most can.  If you are not interested in that part of the hobby OGR has a subscriber, Maine benchwork, that has what looks to be a great product.  If I ever rebuild my layout in a new location that is my first choice.  

You will quickly come to learn O scale trains make noise when they run and not the kind of noise you always want to hear.  Some use a product called Homasote, an evil product that sheds all kinds of fibers when cut but also stops rail noise in its tracks (pun intended)  The good thing about it is once you have placed it you are done with it forever.  I have used homasote and 2 inch pink insulation.  The pink stuff is easier to handle and can be carved, stacked etc.  Start with the book, then ask away, I am sure others will chime in.

Michael,

The two types of framework used most often in model railroading are the box frame and L Girder. Since you have two tables pushed together to support your layout, the box frame would be the better choice for you. Here is a video Lionel produced on making a 4' x 8' box frame. The size of the frame you build will depend on length and width of your tables. If you choose to go wider than 4'. don't forget to add legs to the middle of the frame for added support.

Note in this video that Lionel adds a sheet of homasote to reduce the sound. Other modelers use 2" pink foam that come in 4' x 8'. Depending on the type of track you use, roadbed (cork of my favorite, Woodland Scenics foam roadbed) will also help reduce the noise.

Hope this helps!