First of all this is quite normal and the earlier it's detected the better. I had gestational diabetes with my first and was borderline with the second. Both were controled by diet and exercise. Although you may need to control it with medication or inculin shots.
Your ob/gyn should have already had you schedule an appointment with a nutritionist. The biggest part of this is changing the way you eat. You'll have to cut out most sugar from your diet, and limit the quantities of many types of food. The nutritionist will also recommend eating several small meals/snacks thoughout the day. I ate 6 times per day: breakfast, mid-morning snack, lunch, afternoon snack, dinner, evening snack. Of course, you'll be drinking gallons and gallons of water.
You'll also have to check your blood sugar several times a day. It's simple, just poke your finger with a special lancet and put a drop of blood on a strip and insert the strip into a machine. HINT: Always alternate fingers. If you wrap a small rubberband around the end of your finger you won't have a problem getting a large enough blood drop.
This will also be considered a "high risk" pregancy so your ob will be more cautious especially near the end. Expect to have a few more test than your first.
Most importantly.. Don't worry.
Best of luck to you.