Exactly it will take a long time to learn the correct technique with a trainer so don't even really think of trying to teach yourself as bad habits develope and die hard. Mass building is pretty easy, the key is high weight low reps, eat a lot of healthy food (a lot of protein) and rest a lot with crap all cardio. Do that and you will be gaining a lot, boxing however will make you put on some muscle, especially if your that skinny. But find a trainer and get into it, best fun you can have while exercising and pushing yourself to the limits.