When I was diagnosed, my A1c was somewhere around 11. That's bad. Really fucking bad.
Went on drugs, cut our carbs and got it under control, but I was slowly needing to increase dosages.
Went on a basically zero carb diet (meat, veggies, protein shakes) and started running. Lost 100 pounds and got off all meds (type 2 meds and BP meds). A1c came in at 5.3 around then, which is basically normal to really good.
Got divorced and started enjoying myself a little too much, got injured, stopped running, never really got back to it. Ended up in ER with stupid high BP and my A1c was back up too high (just under 7 as I recall). Went back on meds, cut back on carbs and got it all under control.
Back to where I'm looking to add meds/increase dosage to keep it under control. Not real happy about that, so I've very recently bought some new running shoes/shorts and I have to get back it.
If you're borderline, you can probably just make some tweaks to your lifestyle and keep it at bay. Once you've broken your metabolism, it's always sitting there just waiting for you to fuck up again.
Low carb and high fiber work. I've controlled it being a meat-atarian and a vegan both. Neither works forever unless you're also keeping your weight in check and exercising pretty much daily (once you're broken....)
If you are borderline, I'd highly recommend cutting out bread, pastries, chips, pretzels, high starch veggies like potatoes and corn. And take a LONG walk after every meal, at a minimum.
If its new to you (or whoever you're asking for) don't believe me about the carbs. Buy a meter and test yourself after every meal. Keep a simple log of your what you ate and what your blood sugar level was an hour or two after every meal. Your meter will quickly show you what's what.