Ms Lady Raptor is DEAD ON - except for the typo about the muscle and fat.
FAT IS LIGHTER than muscle because it is far less dense.
Fat takes up more space and weighs far less than muscle. That is why I tell everyone who is interested in building muscle and losing fat to NOT WEIGH themselves because if you are doing things right, then you will most likely gain weight. How then do you guage your progress more accurately?
WITH A TAPE MEASURE AND THE MIRROR!
As you lose fat and gain muscle you will undoubtedly become far more compact but not necessarily much lighter at first.
And to answer your question - by building lean mass (doing weight resistance training and employing proper nutritional habits) and adding a sensible cardio plan.
How do you do this?
Reading and learning from those here that are willing to share
and
Altering your diet.
Just to give you an example of how much heavier muscle is than fat - my hubby and I went to the Texas State Fair last night. I always like to go to the "I can guess your weight (within 3 pounds), your age (within 2 years)" booths. I ALWAYS win a prize, matter of fact we won 4 times. My weight was guessed at 117#, 126# and finally 129#. I am 5'3" I weighed in at 135#!!! My age was guessed at 29. I am 40. Here is a recent pic. I realize that many of the women on the board may not want to achieve the level of muscularity that I have but believe me when I tell you that I just know how to pose to make it "seem" like I got "sumthin". LOL
The first and second pics were taken within weeks of one another. My body is the same just different posing/liting.
Please Scroll Down to See Forums Below 
