Just wanted to add to Stefanos answer, for the serial number and machine specs: on an iMac G5 and Intel, look on the bottom of the stand. On PowerBooks look in the battery compartment sidewalls, for MacBooks remove the battery and look inside, it's can be seen in the bottom (actually located on the bottom of the upper case). The Unibodies have it printed on top of the RAM slots. For MacPros remove the door and look on the bottom of the frame opening. For G4 iMacs, open the optical bay door, it's under the flap. For G4 towers, look for a plate on the back of the machine. G3 iMacs had it in the port compartment.
The answer is fairly easy, and the question is useful, because it should be read before any other question.
Go to www.everymac.co
You can find the Serial Number in the System Profiler, that you can access by clicking on the small Apple symbol on the top of your display, on the left.
If your computer doesn't boot, or if you can't find the serial Number in the System Profiler (I helped a friend fix a Powerbook G4 with OS X 10.1.x, and it didn't have the serial number in the Profiler, but it did show up when he installed OS X 10.4.11), usually the Serial Number is printed somewhere on the machine: laptop may have it under the battery, if it's a removable, or under the bottom case, iPod, iPhones and iPad have the Serial Number printed on the case for sure, desktop Mac may have it printed somewhere behind, where it's not a sore for the eyes.
Failing all of that, the original package _always_ includes the serial number printed somewhere.
Furthermore, if you registered your Mac when you got it with www.apple.com, for AppleCare purposes, while the registration process asks you only your AppleID and the Serial Number, your AppleID page will always show your devices listed for model.
Just if you feel a bit lazy, Mactracker, available as a stand-alone freeware or free to download in the Snow Leopard/Lion App Store, has a function that can detect your model, and usually, at least with G4 and Intel ones, it works.