What kind of bird is it? Do you know? Feathers - yes/no.
If the bird is so young it doesn't have feathers, or it only has down etc. The very, very best thing to do is return the bird to the nest if possible.
If the bird had feathers, but looks 'unkempt' it may be a fledgling and you must release it immediately. Fledglings do not always look old enough to be out of the nest, but mom and dad (depending on species) are typically keeping a very close eye on junior. They will warn for predators, and the young birds needs to learn how to fly/escape etc.
If it is impossible to locate the nest, and the bird is young, you can make a temp. nest (cardboard box) and place it in a tree/shrub and watch (from indoors if possible) to ensure that a parent returns.
Only if the above is not possible should the bird be raised by a person. And the person to raise it should be a rehabber. Care varies depending on species, and it is important to minimize human contact for eventual release. Moistened cat food is okay - temporarily - for some species, but not for all.
As someone who has worked in wildlife rehab in the past, please contact a rehabber. They may not be able to help the bird, depending on it's species and how many other animals they are dealing with, but it is much harder to nurse a baby animal to health for release *after* it has been improperly cared for than one that was immediately brought to them.
I know that last paragraph sounded a bit harsh, but I have had to care for animals that were helped by good samaritans (with the best intentions) that inadvertently caused more harm to the animal.