slightly desperate

Guys, I need help losing weight. I'm 5"7' (1.70m) and I weigh 79.5kg (175lbs). I really want to lose weight, but no matter what I try, I can't seem to get below 75kg. According to BMI charts and the NHS and stuff, I should be between around 54kg and  70kg to be 'healthy.' Any tips or things to avoid to help me get there? Please, anything is really appreciated. I'm 16 and I can't really get my hands on anything that's not a parent approved physics textbook, so I can't try fit tea or anything like that, despite me wanting to