You can't really do anything to prevent gestational diabetes as it is caused by the baby itself. It goes away immediately after birth. I had gestational diabetes with both my pregnancies. The OB/GYN suggested a scheduled C-Section because during the last month the baby can grow really big.
You can start a diabetic diet before hand if you wish, but that will not prevent you from having gestational diabetes. It is likely you will have it again a second time. There are a lot of websites for diabetes. I would just google them. That's what I did when I was pregnant. There is a lot of great information. Your OB should also have a pamphlet on diabtes for you as well.
When I was going through the GD, I remember having to eat five small meals a day. Three main meals, with two snacks. Carbs should not exceed 15gm I believe...if I'm remembering correctly, and stay away from starchy foods as starch turns into sugar/carbs.I believe....(A nutritionist will have all the facts for ya, and your OB will probably recommend that you go to one).
Stay away from cokes. Lots of sugar in them. Even the diet ones. The carbination in cokes, and any other carbinated drinks, actually "expand" your stomach leaving you to feel hungry. I learned this from my gastric bypass surgeon.
Best wishes to you! Hope this helps. :-)