"My" budgie is sick and might be dying, PLEASE READ?

My father found a budgie on our doorstep and we haven't found it's owner. We got a cage, food and water for it, but all it does is sit at the bottom of it's cage. It can't fold it's wings so they kind of just hang limply, it has watery droppings and is always closing it's eyes without putting it's head under the wing, like it probably would if it was going to sleep. It tumbles around instead of walking, and all in all it looks terrible. Please please tell me what to do, because although I've only known it for a few hours I want to do everything I can to save it.