Welcome to MacUpgrades Serial Number Checker. Please input your Mac's serial number below - this can be found on a sticker on the Mac. You can also find it in Apple System Profiler: Select the Apple Menu > About This Mac, then (depending on the version of OSX you are running) click the System Report button or the More Info button. This brings up System Profiler (or on some versions another window, on which you should then click "System Report"), and on the Hardware Overview that is displayed by default in the right-hand panel you should see your serial number, and be able to copy/paste it. The procedure does vary slightly between different versions of OSX - for more info about locating your serial number please see here.
You can also launch System Profiler directly, it is located in Applications>Utilities (that is inside the "Utilities" folder that is inside your "Applications" folder).