I want to lose weight but i dont know how to start?

i am considering advice from pro ana sites. i do not want to be anorexic now do i believe that proana is a "lifestyle choice" or that anorexic people choose to be that way. i just wanna lose weight. i am 14, 162 cm, 46 kgs which is 5'4" and 102 lbs. my bmi says im underweight but i would still like to lose about 3 or 4 lbs. i would like an answer to my question please, not a lecture. thanks ij advance :)