Mate ignore all these phaggots giving you gip, they have never been there before.
You most likely have no underlying medical problems. I used to be 117lb (8 stone 6) because I ate nothing and like you lived a sedentary lifestyle.
Hit the food heavily, do not worry about healthy or unhealthy foodstuff. To get me out of your position I would eat 4 full meals a day, drink milk with every meal and on top of that hit fast food like McDonalds multiple times a week, 2-3 protein shakes a day combined with a solid routine and aiming to progress in weight or reps every workout you will shoot up in size if you provide the stimulus.
There are people like us who are just skinny prior to any stimulus. You will really have to push it hard to put on the weight but once you gain the weight it is hard to lose it, and your body's metabolism will adjust, your appetite will go up to hold onto the weight.
Don't worry if you can't lift much at first. I could barely bench 30kg flat when I started, fast forward to now and I can rep out 100kg incline.
You will have to put in graft though. You will have to monitor your diet to make sure you get in enough cals every day, but it is worth it in the end.