Warning! Ignore all the advice you have heard! None of these people are trained wildlife specialists. (Except maybe Saphyre) I am liscensed for mammals but not birds but I will share what I know...
First let me dispell some of the things said earlier, the bird will not die without it's mother if you act fast unless it's severely wounded or dehydrated. Birds don't reject chicks based on scent but more on how long they have been gone from the nest if it's a single chick. Unless you wait a really long time they will take it back the mother WILL take it back. DON'T attempt to feed it anything if you can help it! Feeding can be tricky and should be left to professionals. (See last paragraph...)
Okay, look up on Google "animal rehabilitators" with the area you are from. Find and call one in your area and they will be able to care for your bird. They have the experience to help you and deal with wild animals every day. If you can't reach a rehabber then contact local vets, shelters, or other places and see if they have the number of someone you can call.
Until you contact help keep him/her warm and in a quite place away from kids and pets. Warm a towel till lukewarm in the dryer and place the baby on it in a shoe box or other box. Exchange the towel for a fresh warm one every hour. (I just alternate between the same two towels.) Give it lukewarm water (Only 5-10 drops)in a syringe or clean eye dropper every few hours to keep it hydrated.
As a VERY LAST RESORT...If you must keep it for more than 24 hours and can't contact ANYONE who can help go with the moistened dog/cat food idea. Make sure it's sopping wet and put a very small amount on the handle end of a spoon or other similar object and feed or drop very tiny peices in. Don't force feed and don't feed more than the equivilant of two peices at a time. You won't want to do this for very long as it's not good for the baby but it will keep it alive for awhile....
Also if you have a small heat pad you can save yourself the trouble with the towels. Turn the pad on LOW and put a small towel over it, you may even scrunch it up a bit to make a "nest". Check the pad constantly to make sure it doesn't get too hot. The pad only needs to be the temp of your skin! If it's too hot it may cause dehydration or even burn the baby. You can do the same thing with a hot water bottle or a jar filled with warm water and wrapped in a towel. The last two will save you from having to change the "heat source" so much, only once every couple of hours or whenever it gets cold. Rememer, test all items on your wrist...No hotter than that!
Small animal wildlife rehabber...