What I did before I started building my own routes was to open an existing route that the game came with in Surveyer and see how things were layed out, look at how the signals work, switches, how railyards are layed out and main lines. Only in surveyer can you see things like track and priority markers, track directional markers, figure out how the speed limit stakes and markers work, how Portals work, the supply and demand of industries. See how splines and buildings are placed, how people used scenery objects.
I started out by taking an existing route and modified it, by adding industries to interact with, if there was a single track main line I layed track right next to it and made it a double track main line. Expanding on a route thats already in the game lets you learn how to create and have existing present to use as a guide. Once you feel comfortable modding and creating that way, you'll feel more confident with creating a route of your own from scratch.
Being a beginner, creating a route of your own can be tedious and very time consuming depending how detailed you want your world to be. I still spend a lot of time just modifying existing routes cause I mostly just want to play and have running the trains.