Before doing anything, make sure the bird really does need your help. The young of some species leave the nest before they are able to fly and usually stay out of sight waiting to be fed. So be as sure as you can be that the bird is not being fed because every year lots of young birds are rescued before they have been lost.
If you do decide to foster the bird be prepared for very long hours and a tedious routine for which you are going to need lots of patience. If you can't commit to this then simply report it to one of the animal or bird welfare groups in your area. If you do take responsibility for a very young bird then you will also need a suitable place to keep it, which must be warm and draught free.
If you can't face swatting flies and killing smooth caterpillars then feed the bird finely minced raw meat from the end of a matchstick. You must never feed the bird live food and always kill insects before offering them. Feed them every hour and make sure the crop is full, which you can tell from a swelling at the base of the throat. It is difficult to overfeed a young bird but the most important thing is to feed it regularly. It doesn't need water so you shouldn't try to offer any.
Don't pick it up if at all possible and never try and make a pet of it because it will be independent of you within a month and your job is to give it the best possible chance of survival. Apart from feeding keep the nest warm and clear away droppings as quickly as possible.
There is other advice I could give about care as it gets older but bird care organizations would probably condemn me as irresponsible and remind me that it is illegal to keep a wild bird without a permit. So, I would advise that you you should look after the bird until one of these bird or animal rescue organizations are ready to collect it from you.