1. The first thing you should do after accesing the internet is run a Mac Update. Simply click on th apple logo located top left corner of your screen and click on software updates and it will display you the latest updates for your mac.
2. Luckily there is no viruses for mac because the underlying structure of OS X is different to Windows and is much harder to write viruses for OS X more info
3. Third, Your Mac should be defended by a Firewall. A firewall is a computer program that stops harmfull programs from attacking you on the internet. Simply access your system preferences, click on sharing icon, click on firewall tab and active your firewall.
4. Receiving or Accepting Emails from an Unknown sender can damage or install viruses in your computer. Avoid to share your personal email on the internet, there is a lot of web sites that want your email adress to send you Spam Email.
5. There is really very functional cool websites on the internet, make sure your browser count with all update technologies, make sure you had the last update of your browser to do this go to your help panel in your browser for more info in how to update.