Welcome, burn101. I was surprised too - and not in a good way. I was eating healthful foods, but obviously too much of them.
Here's what I did to get started;
1. Under the weight tab, enter your weight.
2. Under the activities tab, choose your basic metabolism. (For instance, i chose 'Seated, some movement.") Then choose sleep as an activity, and enter how many hours you usually sleep a day. Then you can see how many calories a day your body burns at your current weight. (So if it shows your body burns 2000 calories, if you eat more than than, you'll gain weight; less than that and you'll lose weight.) It will automatically enter your basic metabolism daily, but you'll have to enter sleep daily (easy to do after the first time from the Recent Activities menu).
3. At the weight goal tab, enter your goal weight and date. Then at the bottom of that page, look at the calorie restriction tab - that will show how many calories you need to spend a day (either through eating less or exercising more) to reach your goal by your goal date.
For example, if your base rate is 2000 calories, and it shows you need a calorie restriction of 700 to meet your goal, you can either eat 1300 calories a day or do activities that burn an additional 700 calories a day, or a combination of calorie reduction and exercise. If you do that, you should be at your goal weight by the date you chose.
4. Enter everything you eat into the food log.
5. Enter any additional activities you do into the activity log.
At the bottom of the food log page, you can look at the calorie balance tab to see how many calories you've eaten vs burned for the day. If you've eaten more than you've burned, you'll gain weight. If you've burned more than you've eaten, you'll lose weight.
Losing weight is a matter of calories in vs calories out. On the bottom of the food entry page, you can look to see if you're getting enough nutrients from what you're eating. The proportion of fats, carbs, and proteins varies by diet plan (for example, Zone Diet is 40% carbs, 30% protein, 30% fats, South Beach diet is 28%-33%-39%, Atkins diet is 9%-37%-54%).
Edited to add: Which diet plan you follow depends on what you're trying to accomplish.
Age 61, Height 5'9"
March 2010 145 lbs, Jan 2013 173 lbs
1st Goal 145 lbs 5/25
New goal 140