First off congrats for taking action to get healthier. There's no secret to losing weight.
Focus on full body exercises will help flatten your stomach. Planks (full planks, elbow planks, side planks, hop out planks) are great for working out your entire core including your abs, hips, butt and lower back. V-ups, situps, and leg lift work wonders. V-ups are tough but once you get a hang of them you'll see grear results. I visit ab-workouts-guide.com whenever I need help finding a new core exercises. Combine the workouts above with good cardio exercises and healthy diet and you can't fo wrong. If you any more questions feel free to PM me or reply here.
Good luck and God Bless!