If you're new to OS X Mountain Lion getting started is simple. Just follow the instructions provided in this free lesson.
When you use any operating system, you'll need to know how to open files, folders, and apps. In Mountain Lion, you can open something by clicking it in the Dock or Launchpad, or by double-clicking it if it's on the desktop or inside a folder.
If you're new to Mountain Lion, you may want to practice by doing the following steps:
When an app is open, the icon on the Dock will have a light under it. You may notice that the Finder icon always has a light under it. This is because it's always running, even if there are no Finder windows open.
Some apps in Mountain Lion have a full-screen mode that lets you just focus on one app, with no distractions. These apps have a double-arrow icon in the top-right corner. When you click the icon, the app will zoom to fill the entire screen, and everything else will be hidden. You can then use the keyboard shortcut Control+Command+F to return to the normal view. Not all apps currently support this feature. However, it is used by many of the pre-installed apps such as Calendar, Safari, and iPhoto.