Hi Endre and welcome to the forum.

Some of the best documentation is
here on the forum, take a look in the documentation section.
I think there are also tutorials and walk throughs on the
NWDA site.
Possibly the best way to learn however is by trial and error (hopefuly not too much

) and posting specific questions here together with files and screen shots. The language of terragen is unique and takes a while to learn but as you read the posts of other users here you'll pick it up

Good luck and remember, theres no such thing as a dumb question.
Richard