There is a wonderful website with all sorts of good information -http://www.kitten-rescue.com
which can answer a lot of your questions.
The fleas will need to be treated as soon as is humanly possible. A spot-on flea treatment purchased from your vet is best, but as you cannot get in until tomorrow, you can get a very small-toothed comb and get a lot of them off of baby now. You will need a bowl of warm water to which you will add just a small drop of dishwashing liquid - you don't need much, just enough to make the water a little sudsy. Comb the baby gently and then dip the comb into the water. Dry the comb thoroughly so as not to get baby wet at all and repeat this until you have removed the majority of the fleas.
If you can the baby to eat on her own, great, but if not, you will need to force feed her. Go to the grocery store and buy several jars of human chicken or turkey meat baby food, warm it gently in a bowl of hot water or in the microwave (it should not be at all HOT, warm to the touch is best) and put a little on your fingers. Offer it to Baby this way to entice her to eat.
Additionally, the dehydration is most certainly an issue and one that cannot be ignored, but the baby food has enough moisture in it to help with that. If you can get her to eat, she will also be taking in fluids. A little KMR purchased at Petsmart or Goat's milk from the grocery store may be helpful. Warm it gently, again either in a bowl of hot water or slightly in the microwave. You can get a little syringe from the pharmacy (request that the needle be removed) to assist in feeding, but be extremely careful not to go too fast. The chances of getting food or liquids into the lungs in a sick young kitten are high, so you must go very, very slowly.
She will also need to be kept warm. Make up some rice-heaters or fill plastic soda bottles with warm water. Wrap them in a soft towel and place them in with Baby to help her maintain her body heat.
Best of luck and thank you for caring enough about this baby to help her.