You can use the About This Mac window to find your computer's operating system version, serial number, etc 

Follow these steps:

  1. Choose About This Mac from the Apple Menu by clicking on the black Apple
  2. Click the version text, for example "Version XX.XX.XX". (click a few times).
Your computer's serial number now appears where the version text had been.

You can click on More Info which contains a number of other menus which will provide additional information.