Am i skinny?Watch
How tall are you?
Start working out if you want muscle I suppose.
Your a healthy weight, theres nothing wrong with being a bit skinny ( also you cant make yourself grow taller. thats all genetic )
I wouldn't worry too much about the graph as it is aimed at adults. As you grow in the next few years you'll probably out on more weight but it's good that you want to be a healthy weight. I'm not sure what to suggest to gain weight. Are you a boy or girl btw?
ecolier provided a chart if you take a look at that or you can find a bmi calculator. Your bmi is 16.5, thats quite low. It should be somewhere between 18.5 to 24.9. If you're healthy though its not too big a deal but thats obviously something we cant determine through the internet ( like the above user said it also is a bit different with children/teens ). It'd be a good idea to speak to your GP about it.