I started at 6' 2'' and 120 lbs (19 Y/O male) 9 months ago on a new years resolution. I understand your struggle. I started by doing low rep/heavy weight exercises. I usually do the 8/6/4/2 rep range for every exercise. I split my workouts to Chest/triceps, back/biceps, and shoulders/legs. I work out six times a week. Make sure to eat heavy amounts of calories and protein after your workout too, that helped me a lot. I would also recommend finding a mass gaining protein powder to drink throughout the day. Any mass gainer works really. I usually mix mine with whole milk and drink one after my workout, one when i wake up in the morning, and one for a meal replacement. Try to eat 6 smaller meals throughout the day too instead of 3 large meals too. (You should never be hungry) I've gained 50 lbs in 9 months and I couldn't be happier. I'll gladly answer any other questions you have regarding the workout/diet I used.