If this is a "plain" 240 volt circuit (no neutral) then you cannot get 120 volts out of it. You may not use one hot and ground as hot and neutral for your new 120 volt circuits.
In addition, if you tapped off of a 50 amp circuit (or any circuit breakered at more than 20 amps) you must go through a subpanel with 15 and/or 20 amp breakers protecting your "sub-branch" circuits for lights and receptacles.
In addition, if you tapped off of a 50 amp circuit (or any circuit breakered at more than 20 amps) you must go through a subpanel with 15 and/or 20 amp breakers protecting your "sub-branch" circuits for lights and receptacles.