Restrict Websites Using My WiFi Router

I own an internet cafe that has 20 computers. Computers get viruses often because most of my customers do not understand the risk in security of most sites like for example, porn sites. What can I do with my wireless router in order to restrict access to some sites I know contains a malware or prevent pornographic sites from getting accessed to.
I want it to be centralized in my WiFi router so that I can make changes on-the-fly.
