For composition gst returns, you have to file GSTR 4. Just download the offline utility from gst site. Fill outwards supplies and upload to the gst portal. Make payments through challans and file your returns.
GST is to be paid on total sales turnover including exempted sales(till 31dec 17) at 1% for normal case, 5% for restaurant. From 1st jan18, you can pay gst only on taxable sales.