IMO mice are foul pets. They bite, they stink (albeit females are less stinky) they poo and pee on you.. etc.

Rats, however, are awesome pets! Much smarter, friendlier, less stinky, and cuter (IMO)

If she is set on a mouse.. get a smallish wire cage (tight wire spacing or you won't have a mouse for long) Feed 1/2 high quality dog food (Nutro brand 'lite' formula) and 1/2 mix of puffed rice/wheat cheerios, oats, and tri color pasta. Water with a bottle. Bedding - aspen is good, carefresh, yesterday's news.. etc.