You weren't born yesterday. You know what a firewall is. You know what anti-virus software is. You know what a man page is, and you've Googled more times than you can count. Let's keep this simple, shall we?
Ubuntu includes ufw (Uncomplicated Firewall) by default. And by default, it is turned off. Turn it on:
$ sudo ufw enable Firewall is active and enabled on system startup
Congratulations. Your computer is now wrapped in electronic cellophane.
Most viruses are still targeted at Windows, but the paranoid can never be too careful. Here is the version for those with short attention spans.
$ sudo apt-get install clamav
$ sudo freshclam
$ sudo clamscan / -ir --exclude-dir=^/sys --exclude-dir=^/proc --detect-pua
If you'd like a GUI, check out ClamTk.