wiring switch and plug to flourescent fixture
Because switches don't take neutral (which you call common).  Only hot wires go to switches.  That thing you think is "the other one from hot", is not neutral.  It's switched hot.

First, connect all the grounds/earths.  

Second, connect the neutrals to each other. 

Third, take the switch out of its retail packaging /shipping box. .   

Fourth, you should have two hots left over, one from the cord, one to the ballast. Those two go on the switch.  

If you want to get some colored tape and color-code (which normal wires are not), then put red tape on the black (non-neutral) wire from the ballast, and tape the two wires on the switch so one is black and the other is red.