try to ride it out as much as possible, stay as consistant as you can, and make sure that if you issue any threats, that they are ones you are willing to, and will, carry out. Ie, if you threaten bed, do it, if you threaten stopping her out-of-school activities, do it, and so on.
if you give warnings, stick to them.
and then, once warnings and punishments are carried out, carry on as before.....
you will have a trying time, and from my own experiance with my own girls, you will find this is a lot hormonal and a lot her own frustrations as she really does not understand why she feels lke this.
think of it if you will of your own feelings and behaviours during PMT (if you suffer it).....it makes things slightly more understandable and managable.
my DD1 was a brat from ages 9-11yrs.......I truly did not enjoy that time with her.....she then calmed down, I stuck as consistant as poss, I even wrote her letters telling her how much we loved her but that we were finding a 9yo/10yo/11yo as hard as she was as we were learning how to raise her as much as she was learning how to be that age!
she is now 14, she is lovely again (she has been from 12 in fact), yes we still have arguements, yes we still have some probs, but and arguements now occur while we BOTH have PMT!!!
I am now going thro it all tho with her two sisters.....(one is 12, t'other 9!!). I often liken it to them being cloned by aliens.....
it is hard, and it IS wearing, but it is all just another phase in parenting and childhood, and one you will get thro.
my advice, and one that carried me thro..
KEEP A GOOD SENSE OF HUMOUR, AND A WELL STOCK WINE RACK!!!!