hayaku:
The fastest method is to use the Special Snap Points constraint. This can be assigned a shortcut key, so all you need to do is hit the shortcut key, click on the two end points, and the Special Snap Points appear, and then select the point you want and the wall will be started at that point (if the Snap Points is set to half, the wall will be started at that point, as there is no other point to select). It is helpful to have the Control Box palette visible to use this function. You can also use the Control Box palette to have the Special Snap Points appear along the the entire element, or between intersecting elements (like the two walls in your illustration.
For more information with images, go to Help > ArchiCAD Help, when your browser opens click on the "Search" tab, and input "Special Snap Points" in the search box, in the results list "Special Snap Points" and "Control Box" will lead to detailed information.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David