Hello and welcome to my User Tip
This is is a list of things you need to watch to make sure your Mac is safe
Beginning with OS X 10.6.8 Apple installed a always on anti-virus for OS X based malware, so also with 10.7 and later OS X versions Apple is watching your back.
However it's not covering Windows based malware (doesn't affect OS X) so if your dealing with a lot of Windows files to other Windows users, then install the free ClamXav (10.8 temporality enable Downloads to "Anywhere" in System Preferences > Security> General" if necessary)
Of course like any anti-virus, it's closing the barn door after the horses have already escaped. So you need to take precautions in what you do.
1: Don't install software from unreliable sources, keep your software updated to fix security issues.
2: Install Flash, Silverlight and other web browser plugins from verifiable makers sources, not from questionable sources or sites (like adult etc)
Check for your plugins that they are updated:
3: Since anyone can get pwned at any time, it's best to have multiple backups so you can restore your entire system in a blink. The best is bootable clones and having them time/dated and kep physcially disconnected. Next followed by more immediate backup/restore like always connected/more often connected TimeMachine and even burn disks of your most important files as once burned, they cannot be changed by malware.
4: Don't use any online banking or do anything online your not willing to risk losing. Know that everything your doing online is being tracked by zillions of advertisers, Google and governments on a always on basis. They might be too busy to look at everything your doing, but they have a record in case they ever want too.
5: If you do or suspect you got malware, then wipe the machine from the Recovery HD partition/10.6 install disk first and then restore from a backup that predates when you suspect the malware got onto your machine (so it doesn't "hop" to another writable volume). Or fresh install the OS and programs and restore only files from disks after a scan by ClamXav. Restoring from a bootable clone is effortless and takes less time and brings everything back, also one can boot from a clone drive, but not from TimeMachine.
6: Other things also could be responsible for security woes.
7: Network Security
8: Don't put anything into or use the machine in a manner that your not 100% willing to share with the world if it comes to that.