Use the Section Tool. User manual lists all of the settings vs visual tutorial on usage:
http://helpcenter.graphisoft.com/guides/archicad-20/archicad-20-reference-guide/user_interface_refer...
Consider watching the video tutorials for the basic use of ArchiCAD to save yourself some time. Graphisoft has a variety of them, with these 3 as a starting point:
http://helpcenter.graphisoft.com/videos/interactive-training/training-series-for-archicad-20/archica...
Some users have posted videos of varying quality on Youtube, too, if you just want to create a section. Also, search the Help Center (link) for what you're trying to do.
🙂
One of the forum moderators
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