Starting with track is backwards.
You start with the road name and era.
That determines the scenery and industries.
Available models or your scratch-building skills determines what you can model.
These industries will need a specific track work. This will be the basis of your layout design.
The thing is you already know what your perfect layout will be. You see it every time you close your eyes and imagine running trains. Now you just have to flesh it out. Maybe do some research. Tell us what you see in your mind's eye and we can give some ideas on where to look. It will probably be the trains you saw as you grew up.
Don't buy anything until you have the theme down. If you do, chances are you have bought a shelf queen.
See if there any clubs in your area. Ask your LHS owner who and where they are. You'd be surprised on how fast you learn what you like after you've run trains on someone else's layout. The clubs are also a great resource in learning how to build your scenery, etc. as most people are willing to give you a shove and let you improve their layout. But take your time. You'll probably find that the planning itself is a great, fun adventure.