Usually, wall lines are the first thing we draw on map editor window. However, we have explained points first. Since this is an introductory work, it's probably better to explain things from the simplest object. Simplest object is, of course, a point. Lines are made from points, so there is some logic explaining them after stand-alone points. You can either draw lines or points first, it doesn't matter.
To draw a line, press “Ctrl-L” or do the usual way to “File commands” panel, choose “Insert line” and press what is actually “Insert line” button. Again, you'll see the red box at the bottom of the window, this time it says “Insert line point”. Now you can click on canvas to begin to draw lines.