Continous Chriping/Beeping
Make sure you've removed the yellow battery installation tag if you've just installed it. If your smoke alarm isn't brand new and is powered by 9V batteries, it will chirp at regular intervals to alert you that the battery needs to be replaced. Get a new battery as soon as possible, and replace it according to the manufacturer's instructions in your User Manual.
Battery Changed - Still Chriping/Beeping
First, make sure the battery terminals are in contact with the battery and vacuum the smoke alarm to remove any dust particles. If your alarm has a Hush mode, it's possible that when the battery was changed the button was accidentally pushed. If it's still chirping after being attached to the ceiling, it's probably in Hush mode; leave it alone for 5-10 minutes without pressing any buttons to let it finish the Hush cycle.