You won't really need av protection. Most viruses are made to infect Microsoft Windows.
Mac & Linux computers are unaffected by these, they will get the viruses, but are immune, the operating system is different.
You may get occasional malware, but it is rare.
If you download and pass on files to a Windows user, get them to scan the file before opening it.
Macs don't even have the firewall on by default. I would put that on for a start:
Go to System Preferences -> Privacy & Security -> Firewall tab -> Turn On Firewall (you'll probably need to click the padlock in the bottom left, enter your admin password and then you can changes settings)
If you want a decent free AV program, try this one:
Macs are generally more secure, but Apple can be a bit slack when it comes to releasing security patches sometimes.
if you want to be super paranoid & make it really secure, create a new 'standard' user and log in as that in future instead of the default 'admin' user. That's standard practice for Linux & Unix users.