If you want to create a new polygon then you should use AGSMutablePolygon.
AGSMutablePolygon *polygon = [[[AGSMutablePolygon alloc] initWithSpatialReference:[AGSSpatialReference spatialReferenceWithWKID:4326]]autorelease];
[polygon addRingToPolygon];
[polygon addPointToRing:[AGSPoint pointWithX:-143.963 y:-41.866 spatialReference:nil]];
[polygon addPointToRing:[AGSPoint pointWithX:-157.947 y:-21.940 spatialReference:nil]];
[polygon addPointToRing:[AGSPoint pointWithX:-165.336 y:11.767 spatialReference:nil]];
Regards,
Nimesh