As you noted, you didn't want to repeat your mistakes. Make a list of good and bad points that you recall with your experience in building and operating a layout. Keep that list handy so you can add to it as your recall stuff.
Make another list of what you want in a layout. List stuff like operating mode; DC or DCC. Determine what operating system you want to use; Digitrax, NCE, LokSound etc.
Determine the area you want to model; the desert south west, mountain railroading, open plains or city.
Determine the era you want to model, i.e. steam, diesel or transition period.
Determine what the primary industries are that you want to model. This will be influenced by the area you want to model.
Your industries will primarily support each other, i.e. steel mills/steel handlers, coal facility/coal industries, Oil/industries that use oil. In other words, have a place to deliver products unless you have a staging yard that acts as the outside world to your operating division.
Determine if you want a classification yard and how big. How long of a train can you build in it? Do you need a locomotive facility? Those usually are co-located with the yard.
Determine if you want to run a signal system. That will soak up money and time, but the pay off is astronomical in operations as well as visitor highlights. You can run a mix of interlocking signals and simple turnout controlled signals. The turnout controlled signals can usually be limited to the yard and sidings while the interlocking will be on the mainline and will monitor block occupancy and turnout settings.
Usually a 4x8 sheet of plywood dooms you to a small operation and limits on where your trains can go. No staging yard potential, broadest curve will be 22" radius. It takes up the predominance of the room it's built in.
A shelf layout around the room perimeter provides broader curves, a longer run for your trains, a means to access a staging yard, more industries, easier elevation changes and potential for multilevel and minimal footprint in the room. A signal system would look much better on a shelf layout around the room than on a sheet of plywood in the middle of the room a desk can be fit under the shelf layout for a work area.