How long have you had it? How young was the bird – feathered or no?
If it hasn’t been long, and it’s old enough that it had feathers, put it back where you found it. Unless a baby bird is in immediate danger, you are never helping by stepping in when you find one. Baby birds spend a lot of time on the ground building up their muscles and learning how to move around.
The rumor that a mother bird won’t take a baby back after it’s been touched by humans because of the smell is not true.
Birds are very delicate and your average person is not usually able to keep them alive. Only a parent bird or a wildlife rehabilitator will be successful. If it hasn’t been long, put it back for the parents to find and they will pick up where you left off. Otherwise, like everyone else has suggested, turn it over to a wildlife rehabilitator.
Read this if you’re interested in learning more: http://www.nwrawildlife.org/sites/default/files/FoundBird.pdf