Skype doesn’t launch with OS X and doesn’t appear to have an option, unlike many other programs, to do this.

However, after some searching I did find a reference to user preferences that allow you to modify which programs start with the system. Unfortunately, these were not 100% correct for Mountain Lion, so it still took me some trial and error to find the correct process. Here’s what you do…

• Press  and select “System Preferences”
• Click on “Users & Groups” (bottom left, in the “System” section)
• The current user should be highlighted on the left. To the right, select the “Login Items” tab
• It now lists all the applications that start when the system does. Those ticked will be hidden (click on the screenshot at the side to see my screen)
• Press the + and – button to add and remove applications. To add Skype simply press + and select it from the Applications list.