Are you sure that it's mother is not around? Sometimes they will leave the nest & come back later. If you don't see a nest, place the bird in a small box with a towel or something to keep it warm. Poke holes in the cover of the box. Try to get in touch with Animal Control & see if they can help. Check online to see if there is a bird rescue in your area. Sadly, there's not much you can do for such a young bird. It probably needs an incubator so keeping it warm is the best thing you can do.
If you don't know what kind of bird it is, feeding it the wrong food could be dangerous. A bird that size would only need tiny amounts of food and would have to be fed often.