As a doctor, the diseases I see most commonly are self inflicted because of lifestyle choices/habits. The best advice I could give to someone is thus:

-Be active, find something you enjoy doing that gets you moving and try to do it every day.

-Eat primarily a whole food diet with variety of plant and animal foods. The less processed, the better. The best diet is the one you can stick to, so don't be too restrictive and don't get down when you indulge.

-Don't smoke. Seriously, just don't.

-Restrict alcohol or abstain completely. Don't drink heavily or consistently, getting cirrhosis is not fun.

-Get plenty of sleep. Most people need 7+ hours. Try to be consistent about bedtime and wake up time. Avoid screens for the hour prior to bed time.

-Be social. Isolation is bad for humans.