If the baby was found in a safe place like a nest, mama might return if you put it back ….. however you would need to keep checking to make sure it was ok and take it again if mama did not return. It's mama will care for it but other animals will kill it.
If the baby is getting milk, then depending on how old the baby is, when you look at the baby's stomach, you will see a white band on the abdomen ….. this is the milk in the baby's stomach. You will not be able to see this on older babies
You need to find a nursing mama rat or mouse with a litter of babies immediately. Do not give the baby to a pet store or a breeder because it will be eaten alive. Look for a mama and litter in pet stores, breeders, shelters, rat rescues etc and also try to find someone on social media, craigs list, kijiji etc that can give you a mama and litter.
Baby will need to be fed every 2 hours and has very little chance of surving because it is so young.
These links will let you know how to care for the baby
Baby will need to be rehydrated, kept at a comfortable temperature, and fed formula with a clean, new (never used) tiny paint brush
http://www.rmca.org/Articles/orphans.htm
http://www.afrma.org/orphanrm.htm
You can also contact a wildlife center to verify it is a baby rat or a baby mouse because it might be a baby squirrel etc If it is a baby rat or mouse, then a good rat rescue may be able to help. If the baby is a rat or a mouse, then a wildlife rescue might use the baby as food for other animals.
Squirrel milk replacer would be the best to use
http://www.mcssl.com/store/17128612/milk-replacers
otherwise I believe you will need to use one of the powdered puppy or kitten formulas (milk replacers like KMR) as shown in the cart on one of the above links, the liquid formulas do not have high enough fat levels. Please get this from a vet asap
I do not know how old you are, but I hope that your parents are helping you with this, because it is going to take a lot of help for this baby to survive (if it does it will be a pet and can never be put outside).
Please post pictures
Please let us know what city/ prov or state/ and country you are in because people may know of resources in your area