I think light cardio is beneficial in losing extra body fat, but I think proper diet is essential to build muscle and, especially to lose body fat.
"Proper diet" is a huge topic in and of itself, but in a nutshell...adequate protein for your specifcs (age, sex, activity level), carbohydrates in the form of vegetables and some fruits, and a bit of healthy fats and plenty of fresh water. Total calories are dependent again on age, sex, LBM, and activity level but if you choose nutrient dense foods and eliminate refined, processed food you are off to a good start.
Good luck!
Jess
Live Well