Diet
This shouldn't be a surprise, but a healthy diet will fix most health issues you may already be experiencing or even prevent any you might encounter. Just to name a few; cracking joints, headaches, depression, oily skin, and many many more.
Even your immune system is directly influenced by your diet.
When it comes to beverages, water is supreme. When it comes to food, it becomes extremely complicated to tell what's unhealthy and to what degree. Do your own research.