Try the LibreCAD
User Manual before the AutoCAD manual. Note that it is still draft and may not match precisely the version you installed, but it will be close.
Also take a look at the
wiki. You find more information there and a couple of tutorials.
As far as the rectangle / circle, it depends. If you used the "Rectangle" tool, you will have to "Explode" the shape and then delete the unnecessary line. Then you will need to "Trim" the circle. An easier option is to draw three lines with the "2 Points" line tool and then add an arc with the "3 Points" or other curve tool.