Thanks in advance for any replies.
Short answer: none that do more good than harm.
Use the internal firewall (external firewall is always a good idea too).
Use strong password for your admin account. If possible, have other users run not at full admin. (If it’s a teens machine, this may not be feasible).
Here’s some basic measures to take for security:
http://www.maclife.com/article/howtos/unlock_truth_about_mac_security_risks?page=0%2C2
If they practice safe computing they’ll be safe.
Re: anti-virus/malware
Currently there are no self-replicating, self-transmitting, self-installing viruses for Mac OS X. There is also no Adware or Spyware save for about twelve Trojans. However, ClamXav is a free OS X anti-virus that will identify WINDOWS viruses that you might inadvertently pass on in email or files you might share. I do not run any AV at all. I haven’t for more than eight years. Neither does Charlie Miller, the guy who just won the CANSECWEST hacking contest by hacking into a Mac. As of now, it just isn’t necessary.