hi shweta..you are grossly underweight
.. first of all you need to do some tests like vitamin D3
and B12 to check if the levels are normal or not and then take injections or supplements for the same..start with weight training
exercises to increase muscle mass..eat high proteins
foods like soya, soyamilk, paneer, tofu, dals, sprouts..high carbs foods like potatoes, sweet potatoes, rice, bananas etc.. some foods options are peanut butter sandwich, milkshakes, adding skim milk powder in porridge/ shakes/ curds, honey in green tea.. eat a balance of healthy foods.. avoid junk and fast foods as that would increase only your fat %..drink plenty of water..