If you have a weight goal you are trying to make, tracking your progress is key. You have to know where you are if you are going to create a plan to get to where you want to be. Know your starting point, see your goal, and then create a plan to get there.
When your goal is to lose weight, more data is better than less. The more weight data you have, the easier it will be to spot trends, and to attach those trends to activities, special circumstances, etc. Tracking your weight daily, or every other day, can help you get the information you need. With that data, you will know if you are on track, or if you need to make some adjustments.
One of the reasons this is important is because your body can lose or gain a couple of pounds each day depending on your body's fluid levels. If you can see a pattern or history of minor fluctuations, then you might be able to point to it and determine that the weight you gained is most likely water weight. This is just an example, but if you have the numbers in front of you, you probably won't have to worry about it so much.
Fortunately, keeping track of your weight is simple. All you really need to do is weigh yourself on a reliable scale, and then keep a record of your weight. It's also helpful to note the date, time of day, and any other information that you consider relevant. "I ate a lot of dessert just now," or, "extra cardio the last 3 days," for example.
Whether you are trying to lose weight to slim up, or gain weight in the form of muscle mass, knowledge will help get you where you want to be. In this case, the knowledge is a frequently-updated record of your weight. Start recording your weight and you'll have the information you need to meet your goals.