If you look at most bodybuilding articles, firstly they will tell you that you can’t build muscle without putting on fat – true to an extent, you can’t – and many will tell you to simply completely ignore the amounts of fat you are putting on whilst building muscle because you can cut later.
Firstly, the reason you can’t build muscle without putting on some body fat is because building muscle requires an excess in calories… and given the body is not a precise mathematical machine, there will always be some left over once the muscle has repaired and grown, and that excess – it’ll be stored as fat, unfortunately.
The ideal situation is, to always feed your body just enough calories to continue growing, but, minimizing the fat gains. Of course, you’ll always put fat on, and will continue to do so – but – you should monitor this very carefully, weekly. Monitoring your fat gains with callipers – and monitoring your weight gain with scales – will tell you everything you need to know.
Forget the miracle transformations. They are either rare, or, usually – outright fake. As a rule of thumb, if you gain more than 1lb of body weight a week. Most of this should be muscle gain, and if you put any more weight on than this, then it’s likely you are putting on fat, too.
How much should you eat? Simple: enough to grow. Some people count portions of food, and others count calories. It’d be impossible for me to say how many calories you’d need every day but, you could try 12 calories per pound of bodyweight per day, and add an extra calorie per lb of bodyweight each week until you start growing.
The final important factor is, make sure the bulk of your carbohydrate intake is before your workout and after your workout, for fuel and recovery. The carbohydrates will be likely to be used for repair and glycogen storage then, rather than turn into fat, as they have a tendency to do if not used quickly.