Better now than never.
Start paying right now, and complete the same from current period, ( if pervious year accounts are open pre audit, then finish the same with that also)
As you have not paid service tax at all, means you are still not registered with ST, register yourself now, and pay for current period,
for previous years, you have two choices with some more details,
if you are registered under excise / Excise or service tax by any means , department will find the same under audit, and you have to discharge the liability at audit front at self willing, you can pay the same now with interest and keep the challans in file to satisfy the audit team that you had no intention to evade, but delayed without any malafide intention, unknowingly.
if you have no relation with excise/ service tax, then get yourself registered with ST right now and pay from current period, and let the department come for audit and on their audit front, state your lack of information/ knowledge but pay upfront at the audit period.
as the returns of previous periods are cumbursome and attract heavy penalty for late filing, its wise not to jump in late return filing unless you have got registered under ST in previous years.