When you take your Mac out of the box one of the first things you do is create a new user. The next time you boot your system, OSX will automatically log you into the desktop as the newly created user. This is a great feature for a single user system. Automatic logins are not a great feature if you are using a MacBook or have multiple users on your system.
It’s easy to disable the automatic login feature by using the System Preferences.
Click on your user name and select the Login Options button at the bottom of the list. Then, on the right hand side of the preferences window select Disabled in the list of Automatic Login options. Close the window when you are finished.