KAAK,
I have some experience with this. I'm taller and lighter than you. My whole life I've had to make serious efforts to keep from losing weight. Most of my life, I was around 103 pounds; I've managed to get up to the 140 pound range, and keep it around there for a few years.
Serious weightlifting definitely helps. Unfortunately, I had to give that up. In lieu of that, the best exercise I've found for weight gain is babysitting and playing with a 3 year old a few times a week. If you take an active role in playing with a little kid, you'll be lifting 35 or so pounds of wiggling little kid, in all sorts of awkward positions, a hundred or more times in an afternoon. Take a tyke to a playground, or swimming pool, or junior gym. It's good weightlifting exercise which is also fun, free, and helps you grow as a person.
Diet: Start by eating lots of nourishing foods... a rainbow of fruits and vegetables every day, etc... but don't stop there. Switch from drinking milk to drinking half and half. Make shakes with ice cream and peanut butter and eggs and fruits and protein powder and everything in your fridge. Add butter, mayo, olive oil, grated cheese, etc., to just about everything. Lots of peanut butter sandwiches. Lots of triple cream cheeses. Lots of steak, eggs, and fish. Carbs, too... lots of potatoes, ravioli, tamales, etc. Eat many meals: Breakfast, brunch, lunch, afternoon tea, supper, dinner, evening snack, bedtime snack, etc. Gorge yourself every time you eat. Eat slowly, and chew thoroughly, to digest your food as efficiently as possible. Keep a little bottle of oil with you to sip on, and a pocket full of protein bars to munch, when you are out and about. Keep fatty snacks by your bed for those late night snacks.
Best wishes on getting stout, Bry.