There are (basic) tutorials on Graphisoft's YouTUBE channel. These are great to get you started with finding your way around ArchiCAD,
There are third party online courses you could follow.
Your reseller might offer a course.
I think the basic course to get you going around here takes about 3-4 days and from my experience with interns and new colleagues who took the course, it is really worth it.
For your specific question I would already ask: do you need to model it? If it is just for Building Information, you could put that information in the zone tool part of the bathroom. A 2D representation in plan view could just be a simple dotted polyline. A 3D represenation for presentation purpose something you model with an extra wall on a seperate layer with its own layer intersection group to stop it messing with the rest of your model.
As you may notice, there is no simple answer without more information and things can get complex fast, depending on the question.
Erwin Edel, Project Lead, Leloup Architectenwww.leloup.nl
ArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5