This is how I went about things:
Step 1: Ween yourself off of soda/caffine. This is going to cause headaches, and it sucks, but there is a crazy amount of calories in Soda that people don't realize. If you need to transition to tea or lemonade first, that's fine. But remember the goal to end with you drinking water 99% of the time.
Step 2: Movement. This plays off of step 1 a bit. When you find yourself drinking water more regularly, you'll find yourself making more trips to the bathroom. This is okay. Any excuse to get you up away from your desk for a short period of time is alright. In fact, get a water bottle that you have to fill up regularly to drink from. Does your work have multiple water fountains or fill stations? Use the further one from your desk.
Are you allowed breaks in the day? Try to take a 15 minutes break once a morning and once an afternoon. Walk. Use this time to literally experience the world around you. Walk down a street you've never been. Walk the perimeter of your building. Wherever you can go. Buy yourself a small fan for your desk because you are going to be sweaty by the time you get back. Especially if you live in a warmer place.
Step 3: Portions. Slowly dial back the amount of food you think you need for every meal. Realize that you should only have one "big" meal a day. The other two should be a sort of appetizer. Maybe cut out 1 of your regular sides. If possible sub a small salad for a side in place of those mashed potatoes/corn/coleslaw/fries. Realize that there is no such thing as "anti-calories". Don't just add a salad to your already large meal thinking it will help fill you up, especially if you add to many toppings/dressings to your salad.
Step 4: My Fitness Pal. Counting calories sucks, but thankfully there's an app for that. You tell MFP what you weigh, and what you want to weigh and it gives you an allowance. This will help you realize your problem areas. Maybe that full glass of OJ is a bit too much for breakfast. Maybe that fourth slice of pizza is going to take up the entirety of your calorie allowance so you hold back. Becoming aware of calories is a crazy learning experience. When you realize those snack cakes in the vending machine are 15% of your daily calorie intake... you start to really wonder if those snack cakes are worth it. Then you start making deals with yourself. 'I can have a little bigger dinner if I walk another 30 minutes.' That kind of stuff. MFP allows you more calories in the day.
Step 5: Exercise. Hey, if walking for 30 minutes gets you an extra 80 calories, maybe running will get you an extra 180. You start adding in all your workouts and suddenly you get to eat again. Then you end up with something like this:
Note* This is not the healthiest example. My leftover calories are way too high. I just missed a fair portion of breakfast and then after work I mowed the lawn (166 calories), went for a 20 minute run (302 calories) and a 40 minute walk (181 calories).