First of all, does the baby bird have all/most of its feathers? If it does, if is a fledgling, and it is supposed to be out of the nest. Baby birds fledge (leave the nest) before they can fly. They need to spend some time hopping around on the ground, climbing low branches and exercising their wings until they have strengthened them enough for flight. The parent birds continue to feed and care for the fledglings until they are self-sufficient. They do not stay with a fledgling 24/7, as they have other babies (other fledglings, and perhaps some remaining nestlings) that they are caring for. They also will not approch the fledgling if there is a predator (animal or human) in the area.
If the baby bird is a fledgling, you should put it back where you found it, or in a low bush near where you found it, and make certain that there are no cats or dogs in the area. Do not leave any food near the fledgling, as that could attract other birds and predators that would harm or kill the fledgling. Leave the area, so the parent birds will come back and feed the fledgling.
If the baby does not yet have its feathers, you can leave it in a basket hung on the tree near the nest. The parents recognize their baby by sound, so as long as they can hear it, they will come feed it.
If it is injured, you will need to contact a licensed wildlife rehabilitator. Do not take it to a vet - vets are for pets and most do not have the expertise, nor the interest, to care for wild animals. They also do not have the proper licenses to keep a recouperating wild bird, and they, too, would have to surrender the bird to a licensed rehabber within 48 hours. And do not call animal control - they will destroy the bird.
If it is injured and you do not have a licensed wildlife rehabilitator near you, I do not know what to tell you. I will not post in public how to care for a wild animal, as it is illegal to have one if you do not have the proper state and federal licenses. Also, if it is injured, it will need medical attention that you are not qualified to give. Maybe you can find a licensed wildlife rehabilitator here that you did not know about: http://www.tc.umn.edu/~devo0028/contact.htm
These people have the specialized training to care for the wild animals, and prepare them properly for release. There is so much more involved than just keeping it alive until it can fly, so you can release it. A baby bird that has been hand-fed must be taught and conditioned to recognize and find food on its own. It must be given enough room, in an enclosed area, to aqcuired its flight skills. It needs to have its weight gain monitored and its general health evaluated. And it needs to be handled as little as possible so it will remember it is a wild animal, not a pet, and will be able to survive in the wild upon release. That is why wildlife rehabilitators need to be licensed - they need to prove they have the knowledge, the skills, and the facilities, that will allow them to do all this properly.