What to do when a bird falls out of the sky?

A bird fell out of the sky and landed in my back garden, and it started trying to fly a bit, and moved around, but couldn't get off the ground, and now it's just settled in a corner, and it's still alive because it's moving it's head, but other than that it's staying still and doesn't seem to be able to move. What should I do now? I don't want it to die.