Work in Top view, doesn’t matter if Camera/Parallel projection or Camera/Perspective. I’m using the latter.
Have your plan image (if you have one) or just a plain rectangle flat in the red/green plane.
Make it into a component, and R-click/Lock so you can’t move it by accident.
Make sure you are drawing on the Untagged tag, or Layer0 in earlier versions of SU - LEAVE the Pencil icon in the Tags/Layers panel on Untagged or Layer0 ALL the time until perhaps, later, when you know better what you are doing.
Start your tool - I’m using Rectangle to draw a inside wall. Or you could start with a large rectangle, or irregular exterior wall outline and draw it with the Line tool, then oFfset it by the wall thickness .
Watch for the blue diamond inference marker showing ‘On Face…’ then draw, holding the Shift key down.
That shows a bold magenta outline for Parallel or Perpendicular to edge (or axes, if no edge was hovered over first).
In 2020 and a few earlier versions, tapping the Down Arrow key has the same effect, and tapping it again toggles between Parallel and Perpendicular. If the wall isn’t on axis, then use the Rotated Rectangle tool, and inference to an angled edge for the start - hover the mouse over an angled line until it highlights magenta.
Or perhaps easier for on-axis walls, tap the Up arrow key to make the Rectangle draw perpendicular to the blue axis, after you select the tool, but before you make the first pick.
You should type in the dimensions if you know them. Or if you don’t perhaps turn on Length Snapping in Model Info/Units to snap to a modular size unit that will be close to what you want - say 10mm or 1/2" depending on your units. Big enough not to miss by one or two units easily, but small enough to give your dimensions ‘round numbers’ at a meaningful precision.
Once you start to inference new objects to existing ones, turn off Length Snapping, or it will give you more trouble than it’s worth.
Every time you draw a new object, make it a component, and give it a meaningful name before moving on.