The primary cause of Type 2 diabetes is insulin resistance. The muscles can't take in the glucose and it causes the blood sugar to be elevated. Diabetes can damage you in so many ways: heart disease, circulatory problems, blindness etc. Without treatment it ALWAYS progressively gets worse, so a failure to control your diabetes will cause your health to decline steadily.
Diet, exercise and medication can slow or in the best case stop the progression of diabetes. Exercise, however, is the most effective treatment. Exercise (strength training or cardio - ideally both) REDUCE insulin resistance, but only for 48 hours or so. That means you need to exercise almost every day, or at least every other day for the rest of your life.
The front line medication for diabetes is metformin. It's a great drug with few, if any side effects. With regard to diet, you have to almost eliminate refined carbohydrates (candy, white flour, rice, potatoes etc.) and eat smaller quantities of complex carbohydrates. Everyone could benefit from eating like a diabetic.
Check your blood sugar regularly to see how different foods affect your glucose levels. Anything over 140 is causing damage, so the idea is to keep it under that most of the time. A brief spike in blood sugar won't cause as much damage, so one strategy would be to follow a sweet with 30 minutes of fairly intense cardio. That will bring you blood sugar down fast.
There are lots of resources out there for diabetic information, but you need to talk to your doctor about medication and blood glucose monitoring. The key blood test done by a lab is the A1c, which tells you how controlled your sugars are over a 3 month period. I am diabetic and mine is 5.8 which is in non-diabetic range, but I follow a good diet, get lots of exercise, and take my medication. Basically, my diabetes is in remission.