It's not good to simply pick an arbitrary number, such as 1000 or 1200 calories, because that might not be good for you. You'll need more calories to start off with, then, as you lose weight, your calorie needs will drop.
Safe weight loss is considered to be 1-2 pounds/week. Best is make lifestyle changes, that you can stick with over the long haul, rather than some type of "crash" diet.
Try implementing the food diary, here on MedHelp. When you set it up, you will enter your stats and it will tell you how many calories to consume each day. Once you set up the diary, you enter in the foods you eat, along with how many servings you ate, each day and it tells you how many calories you consumed. You can set it up to keep track of the amount of fats, fiber, sugar, protein, etc.That lets know, at a glance, if you are staying within your calorie "budget", or if there are certain things you might be eating too much of, like fats or sugar. If you need help setting it up, I'll be glad to help.
You can also use the exercise and weight trackers here on MedHelp. The exercise tracker lets you enter the exercises you did, along with the number of minutes, and it will tell you how many calories you burned. It takes 3500 calories to make a pound, so in order to lose a pound, you have to create a deficit. In order to lose one pound/week, you will need a 500 calorie deficit, each day. You can achieve this by eating 500 calories less, exercising 500 calories more, or splitting it and eating 250 calories less and exercising 250 calories more.
Since you have a desk job, there are "chair exercises" you can do that will help you, too. Additionally, be sure to leave your desk for a few minutes, every hour or so and walk around a little bit. I don't know how long your lunch break is, but maybe you could go for a short, brisk walk, once you finish eating.
Yes, there are lots of tools and trackers out there you can use to track your weight loss and calories. Here on MedHelp they have lots of trackers you can use to track your progress. You can use the food diary tracker to track what you eat, a weight tracker to track your weight, and an exercise tracker to track your exercise activities. You can find out how many calories you need per day upon setup in those trackers. I'm not sure which one has it exactly, but for sure the food tracker. It gives you a general idea of how many calories you should be eating to maintain, lose or gain whatever weight you want. I personally use it myself and think it's great.
Briskly walking for 30 minutes does burn calories, just not as much. But if that's all you can do for now, I think it's great. Once you get comfortable at it, you can certainly try adding a couple minutes of jogging into your walks. Remember, weight-loss is a lifestyle change so don't expect any overnight miracles. You can do it. Good luck!
There are lots of handy apps to track calories in and out per day search calorie counter
Dear Amber: I believe an hour of moderate exercise such as walking will burns about 300 calories... So.. it would be my guess if your walking pretty brisk the 30 minutes is probably burning about 175 calories...You can do a web search on how many calories an hour of walking burns and several sites will have the answer. I lost about 60 lbs 25 years ago and did it with a permanent diet change and daily exercise. I walk about 6-8 miles daily.. An hour each morning and an hour each evening. I also counted my calories while losing the weight and tried to restrict my calories to 1200 per day. After losing the weight, I made permanent life style changes and have kept the weight off for 25 years... In my mind that is the only way it can be achieved. Permanent life style changes. Hope this helps..Claytex