Just eat alot of vegetables. Less sugar and carbohydrates. Try exercising more often. Don't stress yourself though. You can take a walk. U will be fine.
Don't beat yourself up too badly over it, I'm in the same boat as you... I don't really overeat, and the things that I do eat, I think out and try to make healthy meals, but I still ended up with gestational diabetes. Unfortunately, it's just something that happens because of all of the weird things that happen in pregnancy. I've learned to just go with it...
That being said, I was advised to count how many grams of carbs that I take in and what foods contain them. When you see your specialist, they'll give you a list of which vegetables/foods are high in carbs, which aren't, and if they're awesome like mine, they'll give you measurements of how much of something that you're allowed to have. For example, potatoes are loaded in carbs, so you have to eat a different vegetable that's low in carbs like green beans if you're going to have two sides, or you have to be careful how much pasta that you eat. My rule of thumb has been loading myself with protein like beef, chicken, salmon, etc., and choosing small portions of sides for myself. And small, healthy snacks throughout the day like fruit cups (I chop strawberries and blueberries for mine, or take a small container of baby carrots to snack on between meals) and that helps keep my portion control and appetite in check. For breakfast, I switch it up or else it gets old. I'll usually toast an English Muffin and spread this organic cherry fruit spread from Smuckers (6g). Or I'll make myself a small bowl of cereal: 1 cup of Honey Nut Cheerios, and 3/4 cup of milk for my morning intake. Or if I'm in a hurry, a banana and a granola bar. Oh, and stay away from fruit juices - you'd think that they're healthy, but they're actually LOADED in sugar. :/