How to take care of a baby song sparrow?

If the bird is fully feathered, it is most likely a fledgling and has left the nest on it's own and it's parents are caring for it. It is normal for birds to leave the nest before they can fly. You need to replace it where you found it, preferably in a scrub or tree so it is relatively safe from predators, and the parents should come back to feed it. If it is unfeathered, then look for a nest and if you can find it, replace it into the nest. Do not worry about your scent being on the chick as birds have a very poorly developed sense of smell and will not reject a baby that has been touched. If you cannot find a nest, get the bird to a local wildlife rehab for it's best chance of survival. Also, keep in mind, it is illegal for you to try and raise this bird on your own. All birds in the U.S. are protected by the Migratory Bird Treaty Act. Look for a rehab here:

http://www.wildliferehabinfo.org/ContactList_MnPg....

http://www.southeasternoutdoors.com/wildlife/rehab...

If truly injured..a rehab is also your only choice.