A1C is now the standard used to diagnose pre-diabetes and diabetes and to continue to monitor progress on controlling blood sugars once diagnosis is made. It is a test which determines the average of blood sugars for a period of a couple months. A normal A1C would be under 5.7. From 5.7 to 6.4 is pre-diabetes. Diabetes is 6.5 and over.
First off, you don't say if you have been diagnosed with pre-diabetes or diabetes, which is determined by your A1C or other tests. Insulin Resistance, as you may know is a characteristic of Type 2 diabetes. But you are absolutely right, that diet and exercise are important ways to manage diabetes. First off is losing weight if you need to do so. Some people who are very overweight find the weight loss alone reduces their blood sugar dramatically. Then I would suggest cutting out sugar and reduciing carb intake. There is lots of information available online and calorieking.com is a good site that lists carb counts for all foods. (you can also get them on labels of prepackaged foods). Many pre-diabetics and Type 2's have learned that carb reduction combined with exercise can keep blood sugar in range for a long time without the need for medication. I recommend testing, more frequently at first to see how different foods affect your blood sugar (we are all different and there is no one standard diet, though all carbs raise everyone's blood sugar to some degree). Your goal should be to remain under 140 two hours postprandially. You want your fasting sugars under 120 and preferably under 100. Send me a private message if you want me to give you a website where you can share the experience of thousands of pre-diabetics and diabetics and get a lot more details.