echo everything said above.
It is very very common for young kittens to pee on bed sheets. The simplest answer is not to let her on the bed until she is a bit older and keep plenty of litter trays about the house so she can always find one.
As she gets older, you can reduce the amount of trays. But remember the house will be alot bigger to her than it is to you.

Have to re-iterate, do not rub the kittens nose in it, I believe this is a training method used for dogs, it deffinatly does not help with training a kitten. If she is struggaling to use her tray, pop her in it after eating and watch out for her assuming the 'squat' position.