"start it now" This has the more general meaning of the two.
"start it off now" This has a more restricted meaning. The general idea is to put something in motion. eg start off a race, start off on a journey, start off an explosion. Something is moving.
"Start off" is typically used either an intransitive verb or in the construct "start someone off". "Start something off" is rather of rare use. There would not be much difference in meaning between "I started the car" and "I started the car off", though the latter seems to connote more strongly the result of the subject's action "start" - in this case, - the car started running.