Fat and embarrassed

People at school tease me about being fat and it's true I am over weight but I don't know how to lose it cause all the thingies online say stuff like exercise (which I do), eat healthy foods (which I try to do but it's hard cause my mum buys food and she's not gonna change what she's buying I've tried to talk with her) and eat smaller portions (which is a complicated issue because my sister was anorexic and my mum's always on high anorexia alert so if I don't finish something because I'm genuinely not hungry she thinks ihave an eating disorder). I don't know what to do anymore. How can ilose weight??