a GB once gave me the must succinct possible advice on how to lose weight: "Burn more calories than you consume". Find a tracker of some sort (MyFitnessPal, as mentioned above, is great) and do that... you will lose 5lbs in no time.
This.
First understand how many calories you have been eating, and how many fewer you need to eat to stay at 20 lbs less. Go to
http://www.calculator.net/calorie-calculator.html and fill it out first for your current weight and then for your goal weight.
For example, 5'5", 35, sedentary life, 200 lbs, would average 2120 calories a day to maintain that weight long term. Same person dropping to 180 lbs and staying there would be eating about 2015 calories a day.
That reveals a first important fact.
It does not take large changes in calories to drop and maintain 5, 10, even 20 pounds. 105 calories is pretty easy to cut each day. A Hershey bar is 214 calories. Snack size potato chips are 160. It's an extra 1 hour walk a day.
Second important fact:
A pound of fat equates to about 3500 calories. If all you do is a 105 calorie change, it takes 3500 / 105 = 33 days for that to add up to a 3500 calorie difference and see a full 1 pound difference on the scale. So you'd only be losing 1 pound a month with that change. You would eventually lose the weight, just it would be really really slow. So during your weight loss phase to reach your goal weight, you probably want to create a bigger calorie deficit each day than just the change you'll need to eventually maintain the new weight.
1 lb fat = 3500 calories.
If you come in 500 calories under your current calorie needs for 7 days, you will lose a pound a week (7 x 500 = 3500 calories = 1 lb of fat). If you come in 1000 calories under each day, you will lose 2 pounds a week (1000 x 7 = 7000 calories = 2 lbs of fat). So our example person at the start of his diet should eat 2120 - 500 = 1620 calories a day to lose 1 lb a week. I say "eat calories" but that 500 calorie difference can come from extra exercise too, of course.
500 calories less a day also isn't a terrible amount to change. And you only have to make that change while in weight loss, not in maintenance once you've lost the weight.. If you just start looking at the label of what you eat, look up online or on menus the calories, you can come up with some changes that will do it. Plus exercise of course. I burned 450 calories in 50 minutes on my exercise bike in front of the TV this morning before work, and I will do the same tonight after work.
Ok, third important fact. Realize that
your scale reading fluctuates a lot based on how much water your body is retaining and how much food is sitting in your digestive tract. Some foods, like starchy carbs, may take 4-5 days to fully pass out of your body. Just yesterday I was helping an iFriend with a diet plan, and he mentioned he's always been able to skip a lunch and lose a pound or two the next day. I showed him the math above, pointed out he wasn't creating a 3500 to 7000 calorie differential by missing one meal, so it wasn't 1-2 pounds of fat loss he was seeing. Instead, stuff he ate days earlier left his body while he didn't replace it with as much food as normal. So the scale mostly was reflecting less food in his digestive tract, not fat loss.
So don't worry about short term fluctuations on the scale. If you didn't create a calorie deficit, the scale change isn't actual loss of fat. If you didn't eat too many calories, a scale gain isn't true weight gain.
I have lost 80 pounds the last 8 months sticking to these simple principles. I find things I enjoy that are fewer calories, that sate my hunger. For me this means some amount of fat in the meal. Don't go overboard on "eating healthy", focus on eating less calories. My brisket, sharp cheddar and bbq sandwich on flatbread that I have several times a week, is fewer calories than are the salads I make, and I'm a lot happier eating them. Find changes you can make and still enjoy enough that they can be part of your long term maintenance once the weight is lost. When I reach my maintenance point, I'll eat some meals more like the old me, but stick with some meals using what I found I liked during my dieting. I use my diet as a test run for what life will be like maintaining my new weight.
Final thing, don't starve yourself. Listen to your body. Keep eating protein. Parts of your body, like your brain, have protein and fat requirements. If you do not provide the protein in your food, your body will cannibalize your muscle for what it needs. That's bad. You don't need to eat less than 1000 or 1500 calories to see a good weight change (using our example numbers). If you want to generate extra calorie deficit by exercising, that is fine. So long as you did also take in enough food for your body's material needs. I exercise a lot (900 calories a day) and there are days I only ate 1200 calories with that. A day or two of that is fine, but if I hit a spot I'm not eating enough for my exercise, I can tell and I just eat more and I'm still losing weight. So the weight loss is a little slower. I'm making a better body and not losing muscle to do it. That's the true goal you should have for yourself.
Best of luck with it GB. Feel free to join Otis's diet thread, people there are fantastic with giving support.