Here are some things that might help:
1) Taking a look at what other people have done and try a hand at modifying the routes. 
This will let you see what others have done and give you ideas on what you can do. I did this and found it extremely helpful almost 10 years ago. 
2) When you're ready to build, start small. 
You can always increase the size of your route as you go and even merge in your route into another one later on if you choose to keep your original bits. Like any project, you don't want to overwhelm yourself with too much at once. This can happen very easily with Trainz. You come up with a great plan, invest in TranzDem (more on this later), download a bunch of content, DEM files, etc. and go at your route building process. Many months later, you're still laying track. Many, many months later, you're still laying track and now adding a few buildings. Many, many, many, more months later, and you're still doing the same thing. You get the idea! Start small and get yourself a route up and running quickly. This will give you a chance to experiment with techniques, learn the track laying and signaling, and play with the AI.
3) Plan, observe, and read.
Look at track plan books either for model routes and for the real thing. This will give you ideas on what you can do. Remember there's no space limitations in Trainz unlike the physical HO model railroad, but keep the modeling techniques in mind (will clarify later). Look at real life too. If you're near a rail line, observe and take notes and pictures of how the tracks go through the countryside. I live on Bing Maps and Google Maps. I'm constantly looking at how areas are laid out so I can build things like that. Now keeping this in mind, even though Trainz is almost unlimited with what you can place on a route, you need to keep things in perspective. The more content you add, the more work your computer will have to do to display the objects. This doesn't count the trains running either. The effect is additive so the more rolling stock you have in your route will also add into performance. So to avoid the stutters too much, try to limit the depth of the scenery to maybe one baseboard on either side of the tracks. You can have more, but then limit the scenery so the most details are closest to the tracks with lesser stuff in the background. Remember the trains are the actors here, not the cows out in the field next to the farmhouse with the super details barn next to it way off in the distance where you can barely see it. You can have scenes like this, but put them closer to the tracks. You can even add in fire hydrants and manhole covers to your street scenes. Put this kind of detail close and let the scenery blend into less stuff in the background.
4) Take breaks from building.
I can't say enough about this. We've all been there. After building and building, and working and debugging routes and sessions, and fixing and building, you get tired and fried. Step away and give your mind a fresh start. I go through months of not doing much other than driving other people's routes researching and doing other things. I've been at this, as mentioned before, for nearly 10 years, so my own route is huge enough to drive at one end while the other end is still grid. I also have another smaller route I've been fiddling with which gives me a break as well. When I've cooled down a bit, the creative juices start flowing and I jump back in again. Like any creative process, give your self a break.
5) Invest in TransDem. 
http://www.rolandziegler.de/StreckeUndLandschaft/startseiteTransDEMEngl.htm
We can't say enough great things about this program. This program, which is payware, allows you to import Digital Elevation Maps (DEM) from the National Map database and create a Trainz route ready for landscaping and tweaking. This $35.00 (roughly) investment is well worth it.
6) Get a First Class Ticket.
The FCT will allow you to download unlimited amounts of assets from the Trainz Download Station (FTP Server). This is about $24.00 per year and pays its self off quickly. Without it you are limited to 100MB a day and a very slow download speed. As a route builder, you want access to the content easily and quickly without having to wait for the next day. Just remember not to go too hog wild - we've all been there and done that. There are so many cool assets up there, you'll want them all, but just remember the more content you have, the more stuff gets loaded into memory and slow things down. It also eats up more disk space and takes longer to backup. 
7) Back up everything, and backup often.
Even though this has been mentioned, I can't repeat this enough. Your Trainz data is everything you'll have - your routes, assets, everything. You lose this and you're out of the water and have to start from scratch. So, back up your route to a CDP and copy your UserData folder to an external backup drive. And to reiterate this again, backups are even more important with SSD drives. They can die very, very, instantly and are not recoverable.
8) Have fun.
Trainz is supposed to be a hobby. Don't let it become a burden. When you're not having fun, take a break. See above. 
John