It's vitally important that you only purchase molecularly distilled fish oil. The process removes any possible contaminants including mercury and PCBs.
Ignore the total milligrams (mg) of fish oil, and focus instead on the combined eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Also ignore overly-hyped fish oils that include worthless amounts of lutein and zeaxanthin just to get the attention of the vision focused patient - you will be paying way too much for the EPA and DHA included in the product.
You want a supplement that contains 500 mg of EPA /DHA per dose or serving. If you're on blood thinners, talk to your doctor about the best dosage for you.
Your choice is basically capsules or a liquid. They're equally effective at delivering omega-3 fatty acids to your bloodstream, so go with the form you think you'll take on a daily basis. Most folks prefer capsules.
The ratio of EPA to DHA used in research varies, but most pharmaceutical grade supplements are made with a 3:2 split. This translates to 300 mg EPA and 200 mg DHA in each capsule.
Any quality fish oil will do, be it from mackerel, salmon or sardines. Do remember that supplements made from algae oil contain only DHA, and those made from flaxseed oil have alpha-linolenic acid (ALA), only a little of which is ever converted into EPA and DHA by your body.
Omega-3 fatty acids can quickly become rancid and lose their power due to oxidation. Look for supplements that include at least 5 IUs of vitamin E per capsule, (as tocopherol, not soy oil), because Vitamin E helps prevent free radical oxidation, in the capsule and in the body.
A few people experience this as their stomachs dissolve the fish-oil capsule. A number of nutrition savvy folks recommend freezing your fish oil capsules to beat the burp. This will cause the fish oil to be released in your intestine instead of your stomach.