I want to get a six pack, how to do it?

Alot of it depends on what you eat, if you eat healthy than you shouldn't have too much fat around the stomach so it is easier to see your abs. In your case, as you said your very skinny, it shouldn't be a problem to get your abs showing! Just do as many as you can and try to repeat it again few times after a few minutes rest. There are also plenty of other exercises you could do though to stop you from getting bored of just doing sit ups. Just search google and im sure you will find plenty of good exercises for your abs!