How to Delete All Browsing History and Cookies From an iMac
You would probably not be accepting of someone looking over your shoulder while you browse the Web. If you use a shared iMac without deleting your browser history, however, that is essentially what happens when someone opens the Safari Web browser after you. After you finish browsing the Web, delete your browsing history and cookies to prevent another person from snooping into your cyber-habits.
Open Safari (click the compass-shaped icon on the Dock).
Click "Safari" at the top of the window.
Click "Reset Safari."
Select the "Clear history" checkbox.
Select the "Remove all cookies" checkbox.
Select each checkbox corresponding to other items that you want to delete, such as the browser cache and saved passwords.