Pick up couple issues of the various model railroading magazines to get some ideas of what you can do. Think about what period you’d like to model. Some like to model the post WW2 era since you can run both steam & diesel locomotives and still look realistic. Is there a railroad you want to copy because you like the way their equipment looks or it is nearby? What kinds of industries do you want to model? Coal? Lumber? Refineries? Passenger? Manufacturing? What type of setting, city or rural? Seashore, forests or mountains or all three?
It’s going to be your world so you can do anything you like with it. Just have fun.

Hope this helped,

Geof from the US