The best way is to eat more nutritious foods.
Eat more whole grains, vegetables, and fruit and less fast food, highly processed foods, and soda.
No pill will ever make up for a poor diet.
Multi-Vitamins are probably the easiest way. Or if you know your lacking a certain nutrient either eat foods that are high in it or take a supplement for it.
A lot of fruits and vegetables and then a good multi vitamin. You should ask your pharmacist about what is the best on the market.