Locking up ubuntu

Hi all, I just wanted to know what’s the best way to lock up ubuntu? Like firewalls etc etc. I have herd a bit about iptable? But am not to sure what they are??

Also, for virus protection? I have always used AVG on my windows computers. I read somewhere that there is a Linux version? Is that any good? Or is there a better one?

And finally is there anything else I need to do to ensure my computer stays clean and secure?

The sticky in this support category contains a ton of useful information. Most, if not all, of your questions will be answered by it.

-Cheers