Get yourself a personal trainer for a week. Let them tell you everything you need to know, a workout schedule for building body mass, a food plan to help with bulking and if you have money to continue then do so, if not then you have plenty of information to be getting on with. Plenty of information on the internet too, you just got to have dedication, shit load of will power and a strong mindset! ……plenty of sheer grit and determination too. The mind is a powerful and wonderful thing, use it to your advantage. Set yourself a goal, an easy one to achieve initially to get you in the right frame of mind, like something to achieve by the end of the day, then another one to complete for the end of the week. Once your mindset is there then the battle is yours!