Service tax is registered and income is below 10 lakh

Service tax is liable on your gross receipts exceeding 10L (for first year). If by way of your comment :"income is below 10 lakh,..." you mean to say your gross amount of bills charged; and there by your said turnover has not exceeded 10 lakhs in this year (and if  in non of earlier years TO exceeded 10 lakhs); yes, then you come under SSP and no need to charge service tax.

But you are required to file nil return.

Even if the firm is registered, but gross taxable value for preceeding financial year is less than Rs. 10 lakh, then for current year NIL returns are to be filed. But the actual value of service is to be declared in the ST-3 return and tax liability is to be shown as NIL, by mentioning the SSP Notfn No. 33/2012-ST.