First of all, your cat is too young to be weaned. Cats should not be removed from their parents until they are a minimum of nine weeks, but preferably twelve and even better if they make it to fourteen weeks with their mother and still getting some milk from her.
Cats should be able to eat dry cat food at around twelve weeks of age. Before that, moistening with some mother's milk (or Kitten Milk Replacer -- KMR) will help them to eat the cat food. Make sure that you are choosing a high quality all life stages diet such as Taste of the Wild for your cat. I have literally saved money with this diet over store brands.
As far as the litter box, cats will naturally use the cat box. They want to dig in order to eliminate, so this shouldn't be a problem unless you have chosen a sub-standard litter or something that hurts her paws.
I know that this is an old question, but if anybody else is stopping by, it might be helpful :)