If you are someone that consumes fish 2x/ week, you are likely getting enough. It has too be coming from a fatty fish source though (bass, tuna, salmon, sardienes, hering etc.). Light andl ow fat fish may not contain as much. They are still excellent to eat, just be aware they may not have the omega 3's you are looking for.
We can also get it from plant sources like flax seed, but this has to be converted by the body to it's usable form, so we tend not to absorb it as well.
Finally, there are actual supplements. If you are not a fish eater, an omega 3 supplement of 500mg is likely a good start once per day.
A couple of warnings though:
Omega 3-6-9 is a waste of your money. We get plenty of omega 6 and 9 from our foods and these are not the heart beneficial omegas. The omega three dose this this supplement tends to be quite low.
Cod liver oil is also a poor choice. It was popular to feed a tablespoon of this to kids back in the day, but cod liver oil can have toxic levels of vitamin A in it. Vitamin A is a fat soluable vitamin, meaning our body cannot filter it out of the body as easily because most areas that do filter vitamins are water based (like the kidneys). So if the levels become too high, it can build up in the liver and cause damage.
Overall, the best option is to ain to eat fish 2x/week. Why even bother with a supplement if you can get the beenfits from food?