First of all, there is no supplement or vitamin that can replace healthy eating habits. So don't think even for a second that you can indulge yourself with a Big Mac and supersize fries and toss in a vitamin or two and still be healthy.
Vitamins do, however, fill in the gaps in any diet. If you don't get enough flaxseed in your diet, then take a flaxseed supplement. Similarly, if you're not eating enough fish, take an Omega-3 fish oil supplement.
Check out this video to learn which supplment s you should add to your day.