hi mate, u seem to come up with a new question every day, which in many ways is good - but most of it can also be answereed by a boxing coach or a bit of light reseach.

first off 5.10 and 11 stone is perfectly normal, and for boxing u wouldnt want to be too much heavier than that, or you will start to struggle with reach etc.

As for muscle building there really is no secret: it comes down to two things nutrition and exercise.

You need to eat big, ie. up your food intake 5 meals etc, if you are not eating every 3 hours your body starts burning muscle, the second is forms of weight training - your 14 so its really not a good idea to be lifting weights etc, for your bones might not be able to handle it etc. However as you asked before about bodyweight exercises they are more appropiate, your sit ups, press ups, pull ups all do a good job.

fundentally u can find all this on the net etc, but at 14 you really shouldnt be thinking of embarking on such a project, its potentially very dangerous, instead get yourself trraining on the jump rope, heavy bag, some bodyweight exercises, up your food intake, eat healthy go for protein, and you'll find yourself getting into shape within months.