I was trying to post some pictures of the popular Norwegian jig that is very effective on Cod.
There is what they call a Lav jig, but you won't find that in any store around here.
A diamond jig will work too. If the fish are feeding heavy on herring, most any jig will work.
A teaser is common to use also, that is attached to a dropper loop about 18" above the jig, not necessary, but probably will improve your catch for the day. I can give you more info on these if you are interested.
As for jigging technique, I will tell you what works best for me...people have their own techniques so to each their own.
I drop the jig to the bottom, reel up a couple turns..then bring the rod in an upward motion, not real fast at a medium pace, and allow the jig to free fall, and keep doing the same thing.
There will be guys jigging around you...just watch what the guy is doing that's catching fish
I use an 18oz and it works for most situations, sometime you can go lighter 12-14oz
When the bite stops on jigs...bait may be the way to go at that point.