Start with a short description or point form note on what you want to accomplish. Add your "givens and druthers" (as described by John Allen, I think). This is you list of everything from scale to minimum radius, room size, era, locale, industries, type of rolling stock you prefer, number of operators you want to be able to host, photography needs, whether you'd like a showcase for scratchbuilt models, or just like RTR, other activities in the room (workbench, etc), trains running in circles, or industrial switching, etc, etc, etc...
Once you get a handle on that, then you can start looking at the requirements for each industry/activity you have picked. That will in turn start to suggest a plan to you. Be prepared for you plan to change somewhat, and for some "givens and druthers" to switch categories
I see one "druther" already - you'd rather build benchwork with modules...
Andrew