I’m a Canadian importer and having a coming shipment from the States. Goods were originated in US.Can I issue & sign a NAFTA Certificate myself as I know all information.

NAFTA Certificate (Form B232 ) or NAFTA Statement of Origin (for goods less than CAD 2500) can be issued & signed by Exporter or voluntarily by the producer (for use by the exporter) from the US’ side.
In you case please ask your US vendor/exporter to do it.You cannot issue a NAFTA as you are not having information about preference criteria.