The first thing you should do is make sure your operating system is up-to-date. Running the latest version and patches will minimize the risk of a virus taking advantage of recently discovered vulnerabilities. You should also update your web browser, email client, and any other services you use to access the Internet. If these programs have an option for automatic updates, enable it. Once everything is up-to-date, install a current antivirus software. There are several free products available for PCs and mobile devices. Keep your antivirus software up-to-date and schedule regular full-system scans, at least once a week.