First you need to check that the bird really is injured or abandoned. In the nesting season, birds that have recently fledged (left the nest) are sometimes found on the ground while their parent(s) are nearby feeding. The bird might look abandoned, but often it isn’t. If the baby is feathered, it’s best to leave the bird alone and watch discretely from a distance to see if the parent birds return. If the parent’s don’t appear, call your local wildlife rescue organisation and give them a description of the bird – they will be able to provide you with further advice depending on the species of bird and where it is.
If you find a bird that has been injured, or really has been abandoned, you need to keep it safe, quiet, still and warm while you seek further advice. The best way to do this is to loosely wrap the bird in a towel and gently place it in a secure and well-ventilated box. Place the box away from noise, pets, children or other disturbances. Do not attempt to feed the bird. Contact your local wildlife rescue organisation who will be able to provide further advice.