Click on the Apple (top left corner) and choose About This Mac. You will see your processor speed, installed memory. Click on the Version line (under Mac OS X) and you will see the build number of your installed version of OS X. Click that line once more, and you will see the serial number reported by your logic board.
Finally, click the More Info... button, and you will open System Profiler, which will show you much more info about your system.