Depending on your age, gender, state of health, and activity level, everyone will find a different answer to that question.
In youth, we should develop the eating, exercise and rest habits that will lead us to a healthy adulthood.  As we age, we need to give more attention to cardiovascular and brain health.  Older age requires more attention to balance activities like Tai Chi, to minimize falls, head injury and hip fractures.
But there are some overall guidelines that apply to almost everyone.
Here are some ideas:
1. Maintain a healthy weight.
2. Eat healthy food.
3. Get enough physical exercise.
4. A healthy mind in a healthy body
5. Variety is the spice of life.

Diets rich in vegetables, whole grains and seafood are generally acknowledged to be healthiest.  But the body is pretty adaptable.
In general, it's probably more important to know what not to eat.
So, stay away from processed food - chips, candy, snack foods of all kinds...anything in a can or box will not be as good for you as something fresh.
In addition, anything we overdo can be bad for us, so you need to organize a way to monitor and control your food intake.

There is no simple answer to this question, but there is overwhelming evidence that remaining sedentary has negative consequences for health.  Many studies show that increasing your exercise routine even a little can be a big help in staying healthy.

Remember - before you make any significant changes to your routine, especially if you are over 40 or have a pre-existing health condition, you should consult a health professional.