You have to work with Groups to keep geometry isolated from other geometry. So after 'finishing' the first shape - double click it to select the full set of geometry (edges + faces). Then right click to bring up the context menu and choose the Group command. You can also use the keyboard shortcut 'G' when you have elements in selection.
At this point you can single click a group to select it - then you can move - or otherwise adjust it.
To edit the geometry once it is grouped you can double click to enter edit group mode.
I hope this helps!
Tobias