Make sure you add muscle and not fat. Use protein.
Why do you have to be at least 50kg? Just being nosy!
Why do u want to gain weight? You are already beautiful :). However if u want u have to eat more proteins ( chicken, fish....) and do exercises with dumbells..
How old are you? You might want to consider not trying to gain weight til your a little older. I was 110 lbs until i was 25. my metabolism barley started slowing down.