Young puppy

Question
Hello!
I've been given a young puppy. I have been told that the dog is around 3 to 4 weeks old. I was wondering,
-what are the things I need to do to replace the mother. I've been giving it some milk but I don't know for how long I'm supposed to give it.
-What are the signs I should check for to know if he's healthy? Aside from a cool and wet nose (well, I believe that's what is it).
-Is it normal for it to twitch a LOT when he
sleeps?
-And my last question (sorry) is... Is there a way I can figure out how old he is.. maybe by the teeth?  

Answer
Get a milk replacer for puppies from a vet or a pet store.  Cow's milk will give the puppy diarrhea. Then get some puppy food and soak it with water to make a mush and see if the puppy will eat it.  As far as health, if the pup is running around, its gums are nice and pink and its urine is clear (not yellow at all) than the puppy is probably OK.