What is the treatment for menarche?

No treatment needed. Menarche is the normal time when the first menstrual period occurs. If the periods are very heavy or crampy, then sometimes birth control pills are used to help this. Painful periods are called dysmenorrhea, maybe that is what you meant? Besides birth control, nsaid's like Motrin or advil (ibuprofen) can be used.
First period. Menarche is the medical term for a girl's first period.
No treatment. If the girl is over age 8 then this is a normal event and as such is not treatable. Under age 8 then future cycling can be prevented with a drug called depolupron.