Welcome from another noob! This forum is a great source of advice, and they are very tolerant of our blundering about. I recommend the video series by Vlad Chopine, known on the forum as Geekatplay. Even though they were made when Terragen was at v3, they are very useful and project-based.
XFrog has a whole page of free samples, but some of them are a bit blocky; I use Blender to smooth out some details, since I use the models in that app as well. I have also learned from this community to use PoseRay (free) to iron out problems with OBJ files before importing them into TG.
This is a fabulous piece of software, and it will reward your persistence. The bruises on your forehead will fade, and you won't even remember banging your head on the desk.