Q: I just bought macbook pro with 10.8.3 os. When i start the system, all applications automatically start up as well! How can I turn them off. I checked the user settings and no application is set tom open automatically!

System Preferences>Users & Groups>Login Items>Unlock the padlock.  Delete all the apps you do not want to start up at log in.  Re-lock the padlock.

May I suggest reading this

http://www.apple.com/support/macbasics/

especially

http://support.apple.com/kb/HT2477

Mac 101

You have 14 days to return the computer w/no questions asked.  Plus you have 90 days of FREE phone tech support on top of your standard 1 year warranty unless you also purchased AppleCare which gives you an additional 2 years of coverage plus FREE phone support.

Strongly suggest that you take FULL advantage of the above before it runs out.  Let Apple deal w/the problems.

In Mac OS X 10.7 and later, any applications that are running when you shut down will be reopened when you start back up again. If you don't want that to happen, uncheck the box in the shut down confirmation window: