Go read the book "How Not to Die". Very good breakdown of how to eat healthy in general and the info is backed by scientific studies.
In a nutshell, not smoking, not being obese, getting at least a half hour of exercise a day and eating healthier (more fruits, vegetables and whole grains and less meat) can reduce your risk of diabetes by 90% and colon cancer by 71%
Also they did a study to see how various groups' blood killed cancer cells. Blood from a control group of sedentary people killed 1-2% of cancer cells. Blood from people who exercised "strenuously" (strenuously is defined as 1 hour of exercise consisting of a warm up and 45-50 min of continuous strenuous exercise including calisthenics, swimming laps in the pool, etc) every weekday for 15 years killed 2000% more cancer cells than the control group. And finally, blood from people who exercise strenuously plus ate a plant based diet killed 4000% more cancer cells than the control group.
So basically planning on getting an hour of exercise 5 days a week every week of your life no excuses, plus eating as plant based as possible is a good first step.