A prime example of scareware is the pop-up that appears on a computer that says the computer is infected with a virus. The user is prompted to click on the link to either clear the virus or purchase the necessary software to clear it. Don't do it. You'll either pay money for something that is completely useless, or worse, you pay money to download malicious software onto your computer.
Make sure you have good antivirus software on your computer. Be familiar with it, and what the interface looks like. Make sure your antivirus software is set to automatically update. Know your firewall settings, and download all security updates for your operating system.