If possible, put the bird back in its nest and/or put the nest back in the tree. The mother bird won't reject the baby because you handled it. Some young are pushed from the nest by the mother because they are sick or defective.
If the bird is feathered and hopping around, a parent bird is probably nearby watching it, and hoping you'll leave it alone. Keep dogs and cats away.
If you're sure that the bird needs help, make it a warm clean nest by lining a small box or berry basket with Kleenex (DO NOT USE AN OLD NEST-THEY CAN BE DIRTY AND HARBOR PARASITES). Put thenest box in a carton or cage; cover at night.