1) Don't visit non-mainstream "singles" sites. (Generally safe, from an internet security standpoint are match.com, eharmony,chemistry.com. The major ones) I am not speaking to the risks of meeting with people/strangers that you find online. This is another subject. I am speaking to the risks posed to your computer security and health if you indiscriminately surf unknown singles sites. These sites have moved to #1 for risk of compromise of your computer.
2) Gambling sites. Moderate risk of accumulating bad guys. Be careful.
3) Visiting Illegal Sites offering bootleg or pirated software are a high risk for spyware and other bad stuff. These sites might be those providing software cracks, cheats, illegal content, scams, spam. Registering and downloading cracks, bootlegs, videos and audios will elevate your risk.
4) Porn sites. Purveyors are from the underside of our culture. They are not always on the up-and-up. We're not making a moral judgement regarding visiting porn sites. it is simply not a good security risk. They are often up to some shenannigans, installing malware or spyware on your machine is not uncommon for these sites. Teenage boys should probably be made acutely aware of this.
5) Never answer yes to a question posed by an unknown source. and as a corrolary, never answer yes to a question you do not understand.
6) Of course Never open mail or attachments from unknown sources. And be careful even if you do know the reported sender. If opening an email attachment triggers popups and warnings, stop what you were doing and do full scans with Antivirus and antispyware software. Remove or quarantine anything it finds. Reboot after scans finish.
This post contains unsolicited general guidelines for safer computing, not intended to apply in all cases. The reader is encouraged to get a second opinion.