Need gst number

Dear Sir/Madam, My husband is running small Rent A cab service.He had taken Service tax number for his firm in November 2015 through one agent. From than he was putting S.tax number on each bill given to Company and put clause in terms and condition that 100 %service tax to be paid by service recipient. But one client paid him some service amount on invoice which he paid to government of around 13000/-. Now since July all customer are asking for GST NUMBER. When he approach to some agent for GST NUMBER for his firm. He asked for Service return details. Since we were not aware of any thing about Service return etc. we have not done any thing since November 2015 only paid received Service tax on our service tax number to government of Rs.13000/- Now the problem is agent is telling you will not get GST number without Service tax return document. Please Help us in this matter we have Shop Act licence of firm and Service Tax certificate of the firm only. And we have to tax new GST number for this firm. Regards, Neha