Do the short tutorial below (Painting Landscape Materials), and then thoroughly read the Landscape Materials doc page. I’m a beginner in landscapes too, and one of the first things to learn is how to generate terrain and paint on it. If it’s too beginner for you because you’ve worked in Terragen and other programs, then I suggest at least thoroughly reading Landscape Materials and looking for a few nodes / tips for how to create slope-based materials and placing of objects.
Painting Landscape Materials
Landscape Materials
“Any network of material expressions can be connected to the Layer inputs in place of a simple TextureSample. This makes it possible to do more complex effects such as transitioning from detail textures to larger macro textures depending on the distance the layer is being viewed from.”