Need to lose weight fast

i need to lose weight fast but there is a catch. i dont want anyone to know im dieting. im 17 and obese i know people talk about me behind my back and want it to stop quickly. meanwhile i dont want people to know because in the past it has made things worse. however this is hard because i am living at home with my mum who cooks for me. i donrt want her to know im dieting.i dont know how much weight i need to lose but i need to do it in about 5 weeks. i need someone that can advise me. thanks