Get hold of Michael Vone's Route Building Guide (about £10 from
Abacus ) which will give you a good grounding in the very basics of building an MSTS route. Then browse the Route Builders Forum, including the Stickies and Announcements, where you will find much useful information.
Also, get RouteRiter if you haven't already, as that has an option to create a new route file by copying all the default route items into a new file.
Welcome, and good luck,
Barry