Birds will not abandon their young if a human touches a baby.
I found a nest on the ground with eggs and/or baby birds inside:
If you find a nest with young birds in it, wire it back up in the tree you believe it fell from. Make sure it is protected from rain or too much sun and try to get it has high as you can. If the nest is broken, take a plastic container (like a butter bowl) just large enough to hold the nest. Poke some holes in the bottom of the container for drainage and securely wire it back to the tree with the nest and birds in it. Watch from a considerable distance for a few hours for the parents to return. If there is no activity after several hours, contact a rehabilitator.
I found a naked baby bird or a baby bird with tiny feathers:
If you find a bird this young, the nest is probably nearby. Look for adults flying near or at you. The adult birds can help you locate the nest. If you find the nest, make sure the bird you found looks exactly the same as the other babies in the nest. If they are alike, put the bird in the nest. Move away and watch. If the parents return and attend to the babies, everything should be fine. If the baby is thrown from the nest again, there may be something wrong with it and a wildlife rehabilitator should be contacted.
I found a baby bird with most of its feathers:
A baby bird with most of its feathers is a fledgling. Fledglings can not fly well yet but will jump to the ground when they are ready to leave. The bird will not remain in the nest or on a branch if it is ready to fledge or leave. The parents continue to feed the youngsters on the ground until the babies learn to find their own food. If you are concerned that a fledgling is orphaned, watch and listen from a distance. Usually the parent will come down and feed it about once an hour. Sometimes the parents are calling it to fly or move closer to them for food. If you are certain, that there are no parents feeding, contact a rehabilitator for advice.
If you are certain help is needed
If you have watched the nest and are certain that the bird needs help, warmth is most important initially. Make a nest from a small bowl or margarine container. Line it with tissues and form a cup-shaped nest so the bird has some support.
Place the nest in a cardboard box lined with a paper towel. Set 1/2 of the box on a heating pad set on low. Cover the top of the box with a towel and contact a rehabilitator. Do not attempt to give the bird food or water without detailed instructions from the rehabilitator. The food or water can easily drown the baby bird. Most get their water from the food the parents provide.