My honest opinion is that you should forget about body somatotypes.
These were developed in the 1940s, I think, as part of a theory of constitutional psychology which has long been discredited but the physical classification element has unfortunately persisted.
Everybody is a combination of all of the types in some way and the link between these and your metabolic rate is just plain wrong.
I'd concentrate on consistent good nutrition, regular and consistent exercise. Compound lifts like the overhead press, squat, bench, deadlift and pull ups as part of a full body split training plan until you reach a decent level of strength before you start to use body part split routines or isolation exercises and making sure you get good recovery between training sessions. Good sleep also. You'll find that you'll gain plenty of mass from this approach regardless of what anybody tells you about body types.
You're in a great place to start with, you're young and lean and provided you train hard and consistently you could set yourself on a path to achieve the physique you desire (within reason of course)
Good luck dude.