Blimey, it should be easy if you start off skinny.
I was skinny when I started doing weights, & now I am getting pretty big man.
You need to be eating ALOT.
3000-4000 calories a day if you are anything like me.Make sure you are eating good foods too, like oats (porridge) fruits, beans, potatoes, vegetables, nuts, seeds, rice, wholemeal bread.I take a pea protein powder which has helped also.Some people on here will tell you to take amino acid supplements, or creatine, but that is up to you.I prefer not to.
Secondly, while you are eating lots, you need to do lots of weight lifting.3 times a week on the weights I think is pretty common around here for people.Lift as much as you can, big weights.
Them after you weights, you need plenty of rest, including lots of sleep.Oh and drink loads of water inbetween meals.
Any questions, feel free to ask