When you are on your way to become fit, you should understand your body, there are three body types; Ectomorphs, Mesomorphs and Endomorphs. Though you may feel that you belong to two of the body types; ecto-mesomorphs for example, you should be able to roughly identify your body type.
Ectomorphs: These are the people who are naturally thin. They have small bone structures and don’t carry much muscle or body fat. They have a high metabolism making it almost impossible to gain weight.
If ectomorphs want to build muscles, their main goal will be to increase daily carbohydrates intake; as well as a high calorie intake overall.
Mesomorphs: These are people with medium-sized bodies; they have solid muscle structure and large bones. They have a high metabolism but not as high as ectomorphs and their nutrition can be high in calories as long as they stay active.
A mesomorph tends to turn food into muscle really easily, hence they can gain lean muscles quickly after a short period of working out and can maintain a low body fat percentage: they need high amount of protein, but calorie intake is less important than that of ectomorph. They can handle a moderate amount of carbs, but if it becomes too high they will be more likely to store fat compared to people with slim bodies.
Endomorphs: These are people who have more body fat and are usually round and have soft musculature. They have a slow metabolism, so they need to watch what they eat in order to get fit. An endomorph should aim for a moderate calorie intake and add cardio exercises and weight lifting so he/she can burn more calories and boost their metabolism.
Building mass for this body type is not very difficult, though one will have to be concerned with loosing weight by being dedicated to his/her diet and workouts, which will help one achieve this goal.
NB: You can change the way your body looks by gaining muscles or losing fat, the rate and degree depends on your body type. The dedication on your training and your lifestyle can bring you close to another body type.