Welcome back. Where you post depends on what you want to post. It appears you figured out how to post a reply, at least; to start a new topic there is a button at the top left of each forum's post list / index page. Thread = topic, at least when referring to a forum.
What cave are you looking for? There is one along the southern edge of the farmlands, but it is empty until you reach the appropriate part of the harvest disaster quest. The dark cave (Ars Magicana vampire kidnapper) is in the north east part of Rivertown, close to the Verdistis access. The cave for the missing cart quest is in the central east part of Rivertown. There is a spider cave in the south west of Rivertown, north of the archer's guild...
