I assume your speaking of popcorn finish ceiling? Check out this link from all the other folks that have had this same question. http://www.hometalk.com/search?filter=popcorn+removal
Damp it with water from a spray bottle and scrape away....there are a couple of specialty scrappers for this purpose with "catchers" for debris. If it's stippled....that's a much more difficult process depending on the mix.