how to install operating system ( Mac Os x ) in apple computer
More aboutinstall apple desktop
If the Apple computer doesn't come with OS X pre-installed, then you install it the same way you do any custom-build PC:
1. Turn on the power. 2. Insert the media for OS X (CD, DVD, USB, etc.). 3. Follow the onscreen prompts.
as listed above is the way to go, when you power the desktop on, just insert appropriate device with the OS on it.
You don't need to insert any media in a modern Mac (just as well as OS X isn't supplied on physical media). Install is done either from the Recovery Partition or from the Internet. Just folow the instructions here: http://support.apple.com/kb/ph13871