If you're overweight how to ensure you don't get diabetes?

You can't but...: First, stop being overweight or obese. That is always harder said than done. Secondly, make sure the proportion of one's diet that is simple starches or sugars is very low. Thirdly, exercise at least three times a week. Those are recommendations for everyone. They cut down on diabetes risk but don't ensure you never get it. As the saying goes, "the only things assured in life are death and taxes."