If the egg is fertile, if it is still alive because the temperature has been maintained, you need to keep it at the correct temperature and humidity while turning it several times a day until it hatches.
Then the hard work begins.
You then need to keepvtge chick at the right temperature while feeding it the correct food (depends on type of bird) every 2 - 4 hours, from dawn to dusk. You then have to teach it how to fly, and feed itself.
If this egg is a wild bird egg and not a cage bird egg, you need to do this without imprinting the bird on you or humans in general, so once it can fly and find it's own food it can be released back into the wild.
In most places it is illegal to keep a wild born bird.