Here's a link to a really nifty calculator for the amount of fish oil you need. The overall formula is from Robb Wolf; The online version is courtesy of Dallas Hartwig.
Why fish oil?
to help to balance the ratio of Omega-3 to Omega-6 fats. Most of us have waaaay too much Omega-6. You need both, but in a 1:1 or 2:1 ratio (O-6:-O-3.) Omega-6's are pro inlfammatory, Omega-3s are anti-inflammatory.
Make sure your fish oil lists the amounts of DHA and EPA contained. General Omega-3 listing? Look for slick tricks like soybean oil. Not good and not the kind of Omega-3 we're looking for.