Start with drinks, many years ago i always make milo almost every morning. Then i start replace with plain water, lunch and dinner all plain water. It's normal that you're not use to it in early stage, but after awhile you be fine. Second stage is slowly stop taking high sugar food or snacks, i mean chocolate bar, lots of candies, kuih, cakes, donut etc.

Most important is your discipline, else your diet be useless.