Hello mstasiuk, welcome to the forum.
Yes, the documentation for Terragen is not great at all and seriously needs to be addressed at some point by the devs. I have a few ideas about how this could be done.
There are a few tutorials you can access online on YouTube. Here is one by Vladimir Chopine on YouTube.
https://www.youtube.com/watch?v=HnRFJ4Vptt8Besides that, the forums is probably the best place to post a question to figure out how to do something specific.
To start off with, I would stick to using the tabs at the top of the screen, Objects, Terrain, Shaders, etc. until you get the hang of the node system, which can get quite complicated. As you start to set up your scene you will see how the nodes work and interact with each other and you will learn to manipulate them directly.