1 pound = 3500 calories.You use calories everyday regardless of exercise (this is your basal rate) ... for me it's 3200-3500 calories/day (it's higher than yours because, well, I'm fatter or taller than you ... the sites below will calculate your basal rate for you based on weight, age, gender, and height - most will also account for your activity level).
Create a calorie deficit (eat fewer calories than you use) each day and you'll lose weight. This isn't easy to do in you head so there are many websites which can help you keep track of what you ate. Some cost money. Some have better databases of food than others. Here's my list of the top 6:
(1) www.thedailyplate.com - free (but upgradable) tracker with excellent (BEST) food database. This is what I use. The reports are not as good as some other free sites (i.e., fitday), but it's only a small difference, really it gives you more reporting information than you'll have time to evaluate. Their food database can be added to and updated by the community ... when someone else enters a custom food, you can search for it and even add to it's description. This is a big part of the reason that they have such a huge database. Their weight tracking display is easy to use and looks nice enough. Great to see that downward slope!
(2) www.fitday.com - free (can pay for downloadable upgrade), great/simple interface, detailed reports, terrible food database ... but this can be made up for by using www.dietfacts.com which allows you to add foods from it's large database to fitday as "custom foods" with the click of a button. However, even with the additional dietfacts foods, the fitday database is much more limited than thedailyplate.
(3) www.sparkpeople.com - 100% free, excellent reports, community feedback, loads of features. But they won't let you set your loss rate higher than 2 pounds/week (I'm aiming a bit above that so this killed an otherwise excellent site for my purposes). They also have detailed reports, many helpful articles, and a very easy, good looking interface.
(4) www.traineo.com - free and easy to integrate into other community groups (many people have this in their "signature" for other forums they visit. Web 2.0 interface is a little excessive at times, but more often beautifully done. Food database is limited, but better than fitday.
(5) www.calorieking.com - this is a HUGE food database that is free and simple to use. However, their tracking and other services are only available if you're willing to shell out $45/year. Many agree that the cost is worth it for this excellent service, though I think there are such good free alternatives available that the frugal choice is to only use calorieking as a supplement to your other food databases.
(6) www.calorie-count.com - free tracker and adviser, some swear by it, but my review of this site is limited. MASSIVE food database, though many foods have limited nutritional facts available. They also have a helpful community support feature.
Other diet site resources:
Lifehacker 'weighs' in (more generally on lifehacker)
Great article by top-foodie Michael Pollan
Calorie-count.com has an excellent FAQ about weight loss here
What does 200 Calories Look Like?
Calories in many brands of beer