If using an email client like Outlook, set it to receive emails in plain text rather than HTML.
Do not open attachments if you don't know what it is or why someone sent it to you, even if you trust the person who sent the email.
Do not open any emails or attachments where the subject line looks suspicious or questionable.
When downloading files from the internet, make sure that the files are coming from a legitimate and reputable website.
Update your anti virus software regularly.
Schedule your anti virus software to do a full scan of your computer at least once a week. Make sure the computer is on when the anti virus scan is scheduled to run!
If you are browsing the internet and see a pop-up message that says your computer is infected and gives you a number to call for technical support, close your web browser. If you can't close the program, force the program to close by doing a "force quit" on a Mac or pressing Ctrl-Alt-Del on Windows, going to Task Manager and selecting End Task to shut down the program.