How do I tell my mom, I want to diet and lose weight?

If she is worried about eating disorders, don't talk about dieting or losing weight. You need to rephrase it and say that you have been learning about nutrition in school and want to start eating more healthy with lots of fresh vegetables and fruits, cutting out junk food and processed food, eating more whole grains and lean protein, etc
If you eat a healthy diet, as opposed to just going on a "diet" you will lose weight because you are cutting out the processed and fatty/sugary foods that cause you to gain and retain weight
Just making simple changes like cutting out all artificial sweeteners, chips, cookies, soda, etc and making sure you are eating at least 5 different fruits and vegetables a day will have you lose weight

Just say "mom I feel like I'm gaining a few pounds and I want to go on a diet and lose weight" there's nothing wrong with losing a few pounds if your doing it the healthy way

It depends. If you are genuinely interested, please give a copy of this question to her and vanish from the scene.