Turn on auto snapping to constrain walls to 90 degrees

Turn on Orthogonal Snapping so the walls and lines you draw stay at 90 degrees, then join walls with Trim/Extend to Corner.

1. Snap settings work like AutoCAD snap settings.

2. With Orthogonal Snapping turned on (Snap Settings, shortcut S N; it is off by default, while Snap Settings Status is on), the walls and lines you draw move only at 90 degrees, keeping the plan orthogonal. Two walls that do not meet are joined into a clean corner with Trim/Extend to Corner (Home or Architecture tab): pick the command, click the first wall, then the second, and both are trimmed or extended to meet; construction lines (Draw > Add Construction Line) can guide where the walls go.

3. Turn it off when you need to draw at other angles.