Tacit is the real expert on this, but I will start the ball rolling with the following:
- The greatest threat to your Mac's security is sitting in front of the monitor and keyboard —ie. you
- Be sure you have a good account password, at least 8 characters, numbers and mixed case letters, no word that is in the dictionary
- Use the same rules for all internet accounts and use different passwords for each internet account
- In your browser preferences, turn off any option to automatically open "safe" files on download
- Any time a web site indicates you need to download something to "better view this site" treat that site and anything on that site as if it is a malware bomb ready to go off at the slightest touch
- If you ever get a popup or popunder or any other unexpected warning that your computer may be infected by a virus and/or offering to "test" your computer for malware the test is guaranteed to be a false warning and the test itself is malware.
- Only download software from sites you navigate to and you know to be reputable
- Do not download any pirated software
- Remember any offer that sounds too good to be true is almost certainly too good to be true
- Install a good antivirus software package, but remember this:
- Much of today's malware is either undetectable by virus checkers or disables the AV software befor it can warn you of the danger
- In spite of the claims of some AV vendors, no AV software can detect a virus until the virus has been recognized and its signature added to the signature database used by your software
- Trojans, scareware, etc.. that are the primary danger on the Mac are installed by the user and are considered applications by AV software therefore not reported
- Store all security information on all your devices in a secure password protected application. Prefereably one that you can synchronize between your computer, iPhone, iPad, iPod, etc. There are several on the market. Two I have used, "Password Wallet" and "Datavault Password Manager" (and others) are available through the APP Store.
- Perhaps most important of all — keep OS X up to date and immediately install any and all security updates
I am sure others will chime in with additional suggestions