How tall are you? normally (as previously stated) they go on a BMI, which takes in to account your weight and height.
If you are anything like me and a skinny build. Get hitting them weights up, its no secret that muscle weights more than body fat. Do not focus on machines, as these only target individual muscle sets. Get doing triceps dips, push ups, pull ups, squats, bicep curls, skull crushers, sit ups, planks etc, however only use free weights. (you can vary on to machines, but they only target certain muscles, and you really want to develop all the little muscles what help the larger muscles function. I am not an expert but it worked treats for me). Get running and mix it up with cycling, do this 3-4 times a week and your muscles will start to increase.
NorthFace