I think i need to lose weight but i dunno how to (woman only)?

Okay I am 14 and i weigh about 49 to 50 kgs and i am 167cm tall... I think I really really need to lose weight.. i want to get down to 46 or 45kgs but i just can't seem too... oh and i am about a week before getting my period and i never really weigh 50kg could you also think my period could have anything to do withh my weight gain... and how can i lose weight??? please answer all mquestionsns