I'm so confused! I'm underweight and skinny, and I want to tone up and build some muscle. (I'm 5'7 & 112 pounds // 175cms & 51kg)
The thing is, I'm scared of getting fat! (I was quite overweight some time ago)

Does anyone know what's best for safely gaining weight? Should cardio and strength trainng be combined? And I hear that you have to eat more calories to build muscle, but how do you know that those calories will be added as muscle and not fat?