Milk !! The doctor told me that my bones would be too fragile if i didn't drink milk ( i hated milk at that time) , so i started drinkin a lot of milk and i took weight ... It's quite weird !!
BUT, i don't advise you to do the same thing. If you really think you are far too skinny go and ask a doctor what to do. But on the photos of you I don't think you are that skinny though !!!
Well try to eat healthy things and to make some sport, i guess you'll start putting on weight when you'll grow older !!