In order to find the calories necessary to maintain your current body
weight, take your weight (120) and multiply it by 15. This equals
1800. Of course, you also need to make sure you are eating the right
things in proportion..i.e. protein, carbs, and yes some fat (good
fat) is important, too.

Next, please remember that you cannot "convert" fat to muscle. You can
build muscle and lose fat, but one cannot convert to the other. (I'm
sure that's what you meant, but I just wanted to make that clear)

Having said that, you will need to make adjustments to your fitness
routine. Cardio activity burns calories and fat, while
strength-training building muscle. Strength-training also helps to
boost the effects of your cardio workouts because when you have more
muscle on your body, fat-burning becomes more efficient.

Hope this helps!