These are pretty much in the order of importance, although, of course, all have importance. Of course, things like good "thinking" help one make better decisions about health, so they are rather entwined, but can be learned in this order.
1ST: PSYCHOLOGICAL HEALTH/HAPPINESS: Feelings, experiencing life as "good"
Fear (Keep only temporary real fear signal capability; debunk all false created fears)
Establish beliefs that work (correct all that don't)
Manage the mind (and understand it!)
Do what creates happiness; reduce eliminate what doesn't
Do what creates fulfillment/satisfaction
2ND: PHYSICAL HEALTH AND ENERGY
3RD: MANAGING LIFE TO PRODUCE DESIRED OUTCOMES (AND SUCCESS)
Principles, Laws, What works
4TH: RELATING TO OTHERS (Vital component, but its quality is determined by what comes first, above)
Learning what works, principles
Screening for quality, right relationships (Partners, friends, associates,,,vital, vital, vital!)
These are the pieces. You have plenty of time to learn these and to implement them to create true happiness.
You need only implement them.
Linking into the sections on each of these, you can then do the learning and follow the plans.