Eat, eat, eat, then workout, workout, workout!
eat 5-6 meals a day. Try to get protein and complex carbs with each meal.
Get a minimum of 1g of protein per pound of body weight,......1.5-2g is even better.
start out going for 2500 calories per day, if you aren't gaining weight then up your calories by 500 per day until you start adding mass.
I would recommend using creatine. You are pretty low in body weight, so you could probably go without a loading phase, or take less than a regular loading phase.
within 15 minutes of your workout take 5g of creatine with 50-75g of dextrose (or grape juice, enough to have 50g of sugars) then about 20-30 minutes later take in 50g of whey protein mixed with water, and nothing else.
Hit the iron hard enough, keep upping your weights when you can easily put out 8-12 reps, and eat like a bull........and you will increase your mass.
you can use weight gainer powders if you really need to, or are in a rush or whatever, but I would recommend mostly whole foods.
Gain! 
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