What this girl is wearing and can you tell me some old british girl costumes?

She's wearing a simplified (for the stage) version of late Elizabethan boy's clothing.

White shirt with red silk (or silk look) breeches, doublet with slashed sleeves, and short cloak.

Silk stockings and heeled shoes.

She ought to have a hat.

There are many websites where you can read about clothing of this period. Here is a whole menu of them:

http://www.elizabethancostume.net/

http://www.elizabethancostume.net/overview.html

She is dressed as a boy in the style of the mid 17th century, around the 1630s or 1640s. Viola spends most of Twelfth Night dressed as a boy, so this costume is quite suitable. it depends though what period your production is going to be set in - some people go for an Elizabethan style, which would have been worn in Shakespeare's day (in which case she should really have a ruff rather than a collar for instance, and padded trunks rather than breeches), and others set it in different periods, or go for just a vaguely old fashioned look. You need to find out what period is being portrayed in your production.