Summarize:

Mac OS X has long been much more secure than Windows, due to the way that the system software has been designed. In the past, less people used Mac OS, which lead to less viruses being developed for Mac. While the population of Mac users has increased dramatically, it is still not as popular as Windows, which is where the majority of virus development happens. The most important use for antivirus on Mac is stopping the spread of malware to other computers. Viruses are very easily spread through email, and while you may not get infected yourself, you can spread the virus to other computers that may not have the same protection as your Mac. Because of the low chance of infection on your machine, there is little need to get a paid antivirus solution. Instead, download a free option that is designed from the ground up to work with Mac OS X. Because the risk of infection is low, you don't necessarily need to constantly scan your system. Instead, use your antivirus program to manually inspect suspicious files and emails. Apple releases security patches on a regular basis to close any exploits that have been discovered. Keep your Mac up to date to ensure that your system is as secure as possible.
Understand the need for an antivirus program. Acquire an antivirus program. Install and run the antivirus software. Update Mac OS X.