If you find an injured bird, carefully put it in a cardboard box with a lid or a towel over the top, and place in a cool, safe place. Birds go into shock very easily when injured, and often die from the shock. If a bird has hit a window and is still alive, it may just need a little time to regain its senses, then may be able to fly away. Do not try to force feed or give water to the bird. Take the bird outside and open the box every fifteen minutues to see if it is able to fly away. If it is still alive after a few hours, you can try to find a local wildlife rehabilitator. Many belong to your state association for wildlife rehabilitators, and can be found with an on-line search.
The Wildlife International website also has a directory of rehabilitators worldwide that may have other facilities listed for your region.
If you have found an orphaned bird, the first step is to determine if it is really orphaned. When many young birds first fledge and leave the nest, they may still have a little down with short tail and wing feathers. Fledglings often also have weak flight muscles and may be fed for a few days by their parents outside of the nest. This is a very vulnerable time for young birds, as they are easy prey for roaming cats and other predators. It is important to keep fledglings safe and to allow the parents to continue feeding them.
If the bird has fallen out of the nest prematurely, or if a tree was cut down and a nest of young is found, a rehabber may be needed.