1. Calories in < Calories out. This is the only formula that matters when it comes to losing (or gaining) weight. Use a site like
thedailyplate.com or
fitday.com to track calories consumed and burned. Of course, it's impossible to know the exact number of calories an activity burns or a food contains, so weigh yourself weekly (daily fluctuations don't matter): if the scale is going in the right direction, you're estimating correctly. If it's not, you need to tweak your estimates.
2. Maintain lean body mass (LBM). The formula in #1 only ensures you'll lose weight, which will consist of not only fat, but also muscle. In a simple sense, muscles are metabolically expensive to maintain, so during a caloric deficit, your body may be tempted to get rid of them. There are several things you can do to discourage this and instead force your body to burn up primarily fat. One of the best tools for this is weight training: doing a full body routine a few times a week using heavy, low rep, compound movements is probably the single best way to tell your body that you need to keep your muscle mass. In fact, if you're a newbie to weight lifting, you might even gain some muscle mass while losing fat. Cardio (running, biking, swimming, etc) can also help in this role, but it doesn't stimulate muscle growth to nearly the same extent, so the primarily role of cardio is as an efficient way to burn calories - that is, cardio increases the "calories out" portion of the equation in #1. However, you'll want to be careful to not overdo the caloric deficit: a deficit of 400-700 calories per day is usually appropriate. If you go much higher than that, a larger percentage of the weight lost is likely to be muscle. Finally, keeping your protein intake high will also help maintain LBM: aim for ~1g per pound of bodyweight (the sites mentioned in #1 automatically track protein intake for you).
Follow the two simple steps above and you'll do just fine. There are a few other factors you'll often hear mentioned, such as reducing carb intake, avoiding high GI foods, and using high intensity cardio, but in all honesty, these will have a VERY minor affect on weight loss for most people. All of these extras only really become important for those that already have a very low body fat percentage (< 10%) and are battling to burn of the last few pounds. If you're the average person, #1 and #2 will be all you need.
