A food diary is an excellent first step to losing weight. Not only does it show you what you eat, it can also help you understand why you eat the way you do, which is often just as important when it comes to long-term weight management. (For example, if you find that you always hit the vending machine after your afternoon meeting, you may eat in response to stress.)
Simply pick up a spiral notebook that's lying around the house and record everything you eat and drink for at least one week. The most important factor in keeping an effective food diary is to keep it honest. That includes recording "tastes" and samples as well as unexpected treats (like a "free" donut from the break room!). Many of us eat and drink without being truly conscious of it. Keeping a food diary is the only way to get an accurate idea of what you take in to your body.