If you live in Karachi, go to GariParts in DHA Phase 2 he will have authentic NGK and Denso plugs. He will have catalogues of both Denso and NGK, there you can see which sparkplugs are compatible with your engine, in your case R18 (which is a petrol engine, thanks to saad bhai for telling us).
Denso plugs are IK20TT.
These are the NGK plugs (you will find cheaper alternatives too)