I have taken vitamins and supplements all my life, starting with NHS orange juice and cod liver oil and rose hip syrup when I was baby lol.
I know it has paid off, I have lifelong interest in preventative health and have a diploma in Nutrition.
The best thing is to read books such as an essential guide to vitamins and minerals, you can get then on Amazon for pennies or visit your local library and look in the health section.
Taking cod liver oil is a good idea in Winter but its best to switch to fish oils in the summer months, getting at least 20 mins sunshine a day without sunscreen is the best way to absorb Vitamin D
http://www.sunshinevitamin.org/ Don't wash or shower straight away after being in sunshine as this can effect absorption.