i want to lose weight... i am a female i want to lose 20 kg....

i want to lose weight... i am a female i want to lose 20 kg... i am 7okg now

i have very low metabolism... i once went to do a check up... the result is without fat... i am 59 kg... bone plus muscle... omg that heavy right... i dun eat a lot... lesser then a lot of ppl... i drink 6 cup of water daily... dun eat junk food ( ok once in a week) or smoke... but i did not do any execerice... only light exercise... cause i am afrid i will build up more muscle... what should i do?????