So you are looking for the fit body...!!
Fit body or fitness, it depends on how you live. Your living lifestyle, is it healthy or unhealthy ? Just because you are slim & not fat doesn't mean that you are fit or if you are fat means you are not fit.
Some people look fat or slim, that may be their natural body built. To stay fit, you must maintain a well balanced lifestyle. You need to do at least 3 times exercise in a week.
You need to eat well balanced food too. Variety of food fruits (even if you don't like some food). Avoid eating outside food, spicy food & junk food.
Make sure you are getting enough time for walk & which is good for healthy body. Try to play your favorite sports during weekends or in free time, this is the thing which makes your body fit.