G'day,
It is difficult o provide answers to all of the points you raise, however with regard to co-ordinates an excellent start is to read through the article here:-
http://wiki.librecad.org/index.php/Coordinate_systemIt not only explains the Cartesian system, but gives good examples of the various forms of entry.
You can input most of these into the command line after activating the line command.
http://forum.librecad.org/file/n5707517/manual.pdf also has an excellent approach to the basics, well worth looking at.
If you can spend so time "playing" around with these I feel certain you will become quite proficient.
Good luck with Librecad, I spent many years with AutiCAD and IntelliCAD and find Librecad really easy to use, you will also if you can spend time on the basics.
Hal.