How do I tell my mom that I don't want to get a circumcision (girl)?

Mutilation - illegal. Female circumcision is mutilation and thus illegal in all civilized countries. Amputation of the clitoris is part of this operation, which should be called by its true name "mutilation". Any physician or quack performing such a procedure should be reported to the police, it is a major felony. Unless this is a bad joke on behalf of your mother.
Female circ. In the United States i would think and hope this practice is not legal.