Sergio,
SketchUp has local terrain capabilities built-in, using Google Earth topo data and imagery. The subject of terrain building and "painting" can take one-or-more chapters of a book! (even though it's barely mentioned in "SketchUp for Dummies.") In SketchUp, it requires combining several tools, layers and features, so it can't be taught with just one URL link/reference.
I would expect that there are good online SketchUp video resources, available from here...
...which includes YouTube and downloadable videos.
If you need more topographic detail than that provided by Google Earth, SketchUp can import USGS DEM data.
Any terrain surface can be "painted" with aerial photography, from USGS, Google Maps, or Microsoft (bing) maps:
A topographic map (graphic image, not data) can also be "painted" onto a terrain surface.
All of the above can take a lot of study and trial-and-error to learn. You should start with "projected" texture tutorials...
...combined with tutorials on terrain building:
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I tried my hand at it a couple of years ago:
"Indexing" a graphic image to the underlying terrain is where newcomers run into the most trouble. I have one model that demonstrates it...
...but, I have to warn you, I don't do much "hand-holding" in the tutorial. I assume that the user already has a pretty good grasp of how to use SketchUp's tools.
-Taff