Ok I guess you are gonna use arrow/spear. Ok so what I learned is that the best place to hit and fish is the gills or the eye(s) because without the gills fish won't be able to breath.If you hit it on the eye it will take a while to kill the fish.The fish will feel lots of pain and start swim like crazy and probably go away.So i will say the BEST way to kill a fish to to hit it's gills.
In case you don't have any materials to kill fish:
IF you are in the wild with fish and you don't have a hook,spear,arrow, etc. I will recommend you to use a rock that his big but easy to grip.Hit the fish on top of its head just behind the eyes.Or get a stick no too thick or too skinny just like between those and start whittle it to a arrow then aim and shoot.
Hope this helps! =D