Depending on your age, weight, and activity level, you may need fewer or more calories for basic sustenance. 2000 calories a day for an adult is a good starting point. Knowing how many calories you need will get you started on knowing how many calories to restrict.
Many resources can be found online to help you with the calculations. As always, the best advice comes straight from a professional - your GP or health advisor can tell you just how many calories you need to eat for your weight goals.
If you limit yourself to 1,700 calories a day, don't forget to account for exercise. Though it may not burn as many calories as it seems it does, it does give you wiggle room for eating a bit more. Note that this caloric restriction is only to get you on track - soon enough you won't be counting: good eating habits will be automatic.
Writing down everything you eat every day will bring to light your food habits and what food groups you may be missing out on. Don't forget to include drinks and snacks!
Part of what makes a food diary so useful is that it keeps you accountable and motivated. You'll be forced to take a look at what you're consuming which may be just the catalyst for change you needed. If your opinion of yourself isn't enough, have a friend be your guide. They can help by going over your diary a few times a week to make sure you're on track. Knowing they'll be there to watch over you may keep you from straying off course.
As you keep this record, note days where you made good trade-offs (a low-fat yogurt instead of those cookies) or particularly good decisions. What works for you? What doesn't? What patterns do you see emerging?
If you enjoy sitting down to a big plate full of food, fill the gaps with a tasty salad or steamed vegetables instead, providing it does not affect your calorie control plan. A good trick is to use smaller plates which give the visual impression of a fuller serving.
It's particularly hard to portion control at restaurants. While you certainly can order (occasionally) that basket of fries, eat only a serving size, not a super-sized version.
WebMD offers a pretty great portion-sizing tool for a number of foods, including mixed dishes.
A healthy diet is a lifestyle and lifelong choice, not a month-long project.