4 Answers

A good way to start is to have two healthy people who want to breed.

Eat plenty of Vegemite.

Good health has a number of components, not the least of which is your family's history.  Generally speaking, a healthy, well-balanced diet (I like the "inverted pyramid"), and a sensible exercise program is a good start.  
Good personal hygiene is imperative also.  Clean hair, nails, skin, and teeth/mouth all contribute to good health.
Too much sun is not healthy for your skin; moderation is the key.  Moderation for YOU is not necessarily moderation for the guy next door.  
A healthy mind and spirit are important, too.  Physically healthy with mentally ill (depressed or sociopathic) can be a deadly combination.  Stimulate your mind with challenges and literature, learning something new every day.  Spirit can be nurtured in a number of ways; many people follow a faith or religion (not the same thing) to grow their spirits.  
Get regular medical checkups; avoid drugs, alcohol in excess, and tobacco.
That's a pretty good overview, but you are unique....your specific health plan will not be the exact same as anyone else's.   

 if the person is ill or injured that must be treated first.  Then look at Bob/PKBs answer.