Hello to everyone!I think that I have installed a virus to my mac because safari opens advertising sites all the time and Facebook stucks a lot.What i should do to make my mac as it was before?

The first step is to find out what you have installed. I wrote a little diagnostic program to help show what might be causing these problems. Download EtreCheck from http://www.etresoft.com/etrecheck, run it, and paste the results here. EtreCheck is perfectly safe to run, does not ask for your password to install, and is signed with my Apple Developer ID.

Disclaimer: Although EtreCheck is free, there are other links on my site that could give me some form of compensation, financial or otherwise.

From the Safari menu bar, select

Safari ▹ Preferences... ▹ Extensions

Turn all extensions OFF and test. If the problem is resolved, turn extensions back ON and then disable them one or a few at a time until you find the culprit.