First of all, eat as healthily as possible, avoiding refined, processed foods as much as you can - they're short on nutrients and tend to contain lots of stimulants.
You should also make sure you cook your food healthily, in such a way as not to change the nutrients you eat (steaming, cooking in water, baking and braising are best). Mix up cooked and raw foods as well.
Make sure each meal you eat contains lots of fresh fruit and vegetables with plenty of Vitamin C (such as citrus fruit, cabbage, exotic fruit and kiwi fruit). Even preserved or frozen, they retain most of their nutrients.
Make sure you get enough carbohydrates (bread, pasta, rice, potatoes, cereals and pulses), and preferably wholemeal ones which release their sugars slowly and keep you going for longer.
Don't neglect your meat and shellfish: they're excellent sources of iron. And try probiotic fermented yogurts such as Actimel twice a day to balance out your good bacteria!