While using internet unwanted sites automatically opens how to stop them?

If the unwanted sites open in a new window: use a pop-up blocker. A fairly decent one is included with all major browsers. You can usually adjust the settings in one of the drop-down menu's at the top(in Mozilla Firefox, Microsoft internet Explorer, and Opera). In google Chrome, it's the symbol of the wrench on the right of the address bar.

If you're automatically redirected to the unwanted site: Turn off redirects in your browser settings.