My sister has lots of books on this, but it just gets down to drinking lots of water (I drink too much coffee) and eating organic fresh fruits and vegetables, especially when they're grown on farms in your area. I don't much like fish, but I think it's probably healthier than a hamburger.
After the fuel (food) you put in your body, I would say some daily exercise for those of us who spend too much time on computers at work and at home. There are skinny people who say they don't really need to exercise, but I don't think they're very healthy or strong. Youth covers up a lot of mistakes.
Also, I think every individual has slightly different food needs. I would ask the healthy older people among your relatives what they're eating and what they're doing to enjoy good health.