I have a plane, as you can see below, that I would like to be the floor of a building.
How would I quickly create walls from all of the edges?
In edge select mode (CtrlTab>Edge), select all the outer edges and press EZ to extrude along the Z axis:
If you are using an older version, you can select the boundary edges like so:
Enter edit mode (Tab)
Select all (A)
Press 3D view > Header > Select > Select Boundary Loop: